Strange RPO codes???


Strange RPO codes??? - 01/31/16 12:03 AM

So I have just bought my first monte carlo ss its a 1986 seems to be all stock other then the previous owner installed headers and took out the air injection. Was doing a little research and checking my rpo codes. Most of the codes made sense but I have three that I don't understand.

Can anyone shed any light on why the car would be ordered with these options deleted?? Also how would the cruise function without the vss??
Posted By: Bitflipper

Re: Strange RPO codes??? - 01/31/16 02:32 AM

Are you sure that's the original service parts tag?

My '87 SS has E5Z, not the other two.

The ECM relies on the VSS, I just don't see how it could be deleted without deleting the computer.

Re: Strange RPO codes??? - 01/31/16 02:44 AM

yes positive the tag is original... the car is a Canadian model.. maybe something different for the metric speedometer? when I got the car the check engine light was on.. the codes are for o2 sensor...I know this is missing they switched to headers and did not put in an o2. and the vss was the other fault code I have... have not had the dash out yet to check for the vss...also I noted when driving it home the speedo is in kilometers per hour but seemed to be reading in mph??
Posted By: Bitflipper

Re: Strange RPO codes??? - 01/31/16 03:16 AM

My car is Canadian too, moved it from Toronto to California 3 years ago. I had a KPH speedometer up until I moved here when I swapped the face plate with a junkyard salvage from an El Camino. So that code doesn't reference the Canadian speedometer, good thought though.

Doesn't make a whole lot of sense that it would read in MPH, the only difference with the stock speedos between US and Canada was the face plate on them. Sounds like yours needs calibration.

You should be throwing Trouble Code 24 if you didn't have a VSS. I just checked the parameters for the '87 SS ECM and that code is definitely active.

Your air/fuel mix is going to be whacked without an O2 sensor. How are you passing Ontario emissions even? The computer uses the info from the O2 sensor to control the mixture control solenoid in the carb. You really need one of those put into your headers.
Posted By: Bitflipper

Re: Strange RPO codes??? - 01/31/16 03:53 AM

I just remembered how you're passing Ontario emissions - you're not, the car is older than 20 years, you don't have to. How easily I forgot that after struggling to get back to passing in California.

Everything else I said stands.

Re: Strange RPO codes??? - 01/31/16 06:43 PM

hey thanks bitflipper. I am getting code 24. where is the vss located? I thought I read somewhere that it was part of the speedometer? Im going to lubricate my speedo cable might just be a little sticky causing the speedo to be off. if not I will try replacing the drive gear. and yes I have an o2 sensor and bung on order I do want to put that back in the system. im going more for looks and fuel economy then full out power.
Posted By: Bitflipper

Re: Strange RPO codes??? - 01/31/16 07:42 PM

Member Monte_ExpreSS covered it pretty well with this blurb:

"Under the dash, near the steering column will be a small, green box. There should be 2 or 3 (can't remember) wires coming off of it heading up in the direction of the speedo. On the end of the wiring is a black plastic optic eye. That is your VSS. it bolts to the back of the speedo & is for the purpose of sending a speed signal to the ECM & cruise, if your car has it. It does NOT control the speedo inself, our cars use a cable driven unit. If the speedo itself is not working correctly, the VSS won't get a reading & therefore show a code 24. Find out what the issue with the speedo not reading is, & the code shoudl go away."

Full thread here: VSS (vehicle speed sensor )

When he says "if your car has it", I do believe he's just referring to cruise control. Your car has an ECM and it is expecting a VSS, your own findings prove that.

You talked about how your speedo reading appears to be incorrect. I'd start by fixing that and see if your code 24 problems go away. Your simplest bet might be to swap the entire speedometer, though you'll have to expect to pay over $100 for a used one unless you get really lucky and find one in a junkyard or something. If you end up having to import one from the US, I can tell you for sure that the faceplate will interchange with your existing KPH faceplate, but you will have to carefully remove and replace the indicator needle which is a PITA because then you have to get it recalibrated (I did it with trial and error, took about 10 tries)

Without an O2 sensor you must be getting terrible gas mileage and your car is likely performing very poorly. I haven't studied what happens once you throw the O2 code, I would guess it runs in Open Loop the entire time, no good for your ride in any case!

Re: Strange RPO codes??? - 01/31/16 08:13 PM

thanks for the info I will check for that box and vss! the car doesn't seem to be horrible on gas used a little over half tank to drive about 3 1/2 hours on the way home. but im sure it could be better.
Posted By: cj84ss

Re: Strange RPO codes??? - 01/31/16 11:22 PM

You will get better mileage and performance when you get the O2 sensor installed and with headers I would suggest a 3 wire (heated) and it will work with the headers much better than the one wire.

Re: Strange RPO codes??? - 02/01/16 01:05 AM

do you just hook up the heater wire to a switched power circuit? never thought about hooking up a heated 02
Posted By: Jameson

Re: Strange RPO codes??? - 02/02/16 02:25 AM

Does the car have cruise control?
Posted By: mmc427ss

Re: Strange RPO codes??? - 02/02/16 04:32 AM

An O2 sensor needs to be 600 degrees to work properly. Headers cool off at idle, plus the O2 is most likely downstream farther, the non heated O2 cools off at idle, ECM goes into open loop. A heated O2 will keep the ECM happy, stays in closed loop at idle by maintaining the 600 degrees.

A three wire O2 is a switched ON power supply and a ground, and of course the third wire is the O2 output. If I remember correctly my power for the O2 heater was taken off the oil pressure switch next to the distributor. Splice into the switch were the electric choke wire is connected. This way the O2 will only get power when the engine has oil pressure, just like the choke does.

A four wire O2 uses the 4th wire as the ground for the sensor, it's run back directly to the ECM board. This just insure that the sensor has a direct path with no resistance to the ECM. The newer cars run 4 wire O2s. When I was running headers and CCC a 4 wire O2 was run, overkill, possibly.

Re: Strange RPO codes??? - 02/02/16 07:02 PM

Thanks bob. Never thought the headers would cool down enough to put it back into open loop. I will definitely look into a three wire and run it off the oil pressure as you suggest. The heater doesnt draw too much power away from the choke coil?
Posted By: Bitflipper

Re: Strange RPO codes??? - 02/02/16 09:42 PM

So far as I can understand (and I have studied it a lot), our ECM (at least the '87/'88 1228079 - I doubt the previous ones were "smarter") uses only coolant temperature to determine whether it is in Open or Closed Loop. O2 reading only appears to be used for Rich/Lean indicator for the mixture control solenoid.

EAAD:   D6 11                       LDAB    L0011 ;	CLCC 1
EAAF:   2A 03                       BPL     LEAB4 ;	BRANCH IF OPEN LOOP 
EAB1:   B6 32 BD                    LDAA    L32BD ;	MIN TEMP CONT CLSD LOOP
EAB4:   91 35               LEAB4   CMPA    L0035 ;	COOLANT TEMP
EAB6:   23 23                       BLS     LEADB

EADB:   96 11               LEADB   LDAA    L0011 ;	CLCC 1
EADD:   84 7F                       ANDA    #$007F ;	OPEN LOOP
EADF:   97 11                       STAA    L0011 ;	CLCC 1
EAE1:   7E EC C4                    JMP     LECC4

Where the thresholds are

32BC:   B9                          FCB     $B9 ;	MIN TEMP START CLSD LOOP
32BD:   C6                          FCB     $C6 ;	MIN TEMP CONT CLSD LOOP

and CLCC is

                            L0011   EQU    $0011 ;	CLCC 1
;	b0 = BLENDED ENRICHMENT FLAG                 1 = BLENDED
;	b1 = COLD START                              1 = COLD START
;	b3 = WIDE OPEN THROTTLE                      1 = WOT
;	b5 = O2 RICH/LEAN FLAG                       1 = RICH
;	b7 = CLOSED LOOP FLAG                        1 = CLOSED LOOP 

Basically that says that the engine coolant has to be reading roughly 52 degrees celcius (about 125 Fahrenheit) to go from Open Loop to Closed Loop. After that, engine coolant must maintain at least 45 degrees celcius (113 Fahrenheit) or it will drop back to Open Loop.

Unless you want to tweak the PROM to lower these thresholds, you're wasting money and time on putting in a heated O2 sensor for the purposes of getting to (and staying in) closed loop. I can sell you (speaking to the general audience) a tweaked PROM that would lower the temperature thresholds, but there is more than what I'm showing here in terms of parameters. There are a few minimum time parameters where no matter what the temperature is the ECM will remain in Open Loop. No one has yet asked for a PROM with these parameters tweaked so I don't know everything it would take to tweak this properly (yet).

Re: Strange RPO codes??? - 02/02/16 10:54 PM

wow bitflipper your one smart that stuff looks like Japanese to but I wondered about that.. I didn't think the 02 heating up any faster would make any difference in an old system like this since the choke controls most of the mixture till it gets warmed up the o2 sensor really wouldn't be needed till its warmed up. also I was wondering if our old ecm's would calibrated the same as the newer fuel injected stuff that the heated ones come from. and I did take the dash apart and I do have a vss in place. haven't had a chance to check the outputs at the buffer yet. got lots of wiring to straighten its got 30 years of back yard wiring. I work as a mechanic for a Chevrolet dealership so at least I have access to all the shop manuals and wiring diagrams...
Posted By: Bitflipper

Re: Strange RPO codes??? - 02/02/16 11:28 PM

Smart possibly, but more importantly, spent far too much time hacking that ECM. smile

I just put the code there incase someone wants to take it up with me. Unless you've done some old systems programming in assembly language, it's hard to read. Heck, it's hard to read period.

The calibration is likely the same for all narrow band sensors, but why chance it; no other type is going to help you, stick with the one-wire one recommended for the stock car. That ECM is waiting for the O2 sensor to be good and hot before using it as input regardless of what kind you put in there.

Bottom line, save your cash for the Speedometer and get a regular O2 sensor. Later on once you have your car running well and trouble codes all resolved, then you can talk to me about a PROM to start doing fancy stuff if you so choose to go that direction.
Posted By: mmc427ss

Re: Strange RPO codes??? - 02/03/16 04:49 AM

Per the Helm's shop manual for a Code 44, "The sensor is like an open circuit and produces no voltage when it is below about 360C (600F). An open sensor circuit or cold sensor causes "open loop" operation." The ECM supplies .45 volts to the O2 between terminal 9 and 14, the ground. By the way terminal 14 is were you would run the 4th wire from a 4 wire O2. The O2 varies the voltage within a range of 1.0 (rich) and .10 volts being lean. Actually it's more in the range of .350 to .800 if I remember, been to long.

The ECM set a Code 44 when the O2 remains below .2 volts for 60 seconds or more. Again, this info from the 1987 Helm's. With the O2 installed in the stock cast iron manifold I doubt the O2 will ever get cold. But installing it down stream in a header collector it should run cooler.

I've installed a couple wide band A/F meters, they all have heaters so once they are up to temp they don't "drop out", but they will need to warm back up even after being turn off for a few minutes on a hot start. Just the nature of an O2 needing to be a certain temp to work.

Years ago I verified this when I ran CCC, headers and had a real time scanner while driving. Sometimes, not all the time but if conditions were right idle would go into open loop.

The old Alltest 3258 Brainmaster was pretty good for reading the CCC system, it's still in a case under a bench at the shop. There are several on Ebay now, a handy tool if you need to run CCC.

Dylan, not trying to step on toes. Years ago another gentleman here did a workup on the CCC programing, I printed it out, like 10 pages of 11x17 paper,I didn't understand the "logic" then, still don't. But do appreciate the effort you put into the CCC. Somewhere in the programming the ECM monitor voltage at terminal 9, (the O2's .45 vdc supply) and is a go, no go for loop status.

There are probably four 079 ECMs laying at the shop along with PROMs for an LG4, SS, ZZ4 EGR delete, and a Caprice 350, collected that stuff many moons ago when I needed CCC to be legal. One of these days I'd like to throw the CCC carb and dist back on the 427 and take it back to the track. Just not sure if a PROM could be burned with a timing curve suitable for my setup. I would guess it's possible to "adjust" the Vac, Temp, TPS, O2 parameters and then take a look at the M/C input from the ECM and tweak the fuel curve a little.

Off topic here, but one thing I did years ago was play with primary metering rods in the CCC. Most all CCC SS Q-jet carbs used the same rod, .026/.056", 75 "large window" jet. A donor DualJet carb yielded a set of .025/.053 rods, and another donor a set of 76 small window jets, which I made large windows. Even bought the primary metering kit from Chevy they sold for the ZZ4 upgrade into the 3rd Gen F, #17087129, it has .026/.056 rods and 75 large window jets, the same parts the SS came with, a waste of money. The donor slightly richer .025 tip along with the 76 jet helped the WOT a tad, and the richer part throttle .053" section helped the transition into the power circuit, until the EGM used the M/C solenoid to catch up to what just happened. The ECM's problem is it's always looking for that 14.8 A/F, except at WOT. That's why you can only fool that slow ECM for a split second with the different rods, but I thought it was something you could feel, the car ran better.
Sorry, got off track.

Posted By: Bitflipper

Re: Strange RPO codes??? - 02/03/16 06:12 AM

No toe stepping assumed, there are 2 kilobytes of code in the ECM which is only partially (and sometimes incorrectly) commented. Perhaps I'm missing something, though I am pretty much firm in my stand that the loop status is decided entirely from coolant temperature, there is no temperature gauge at the O2 sensor.

I agree that the ECM will drop back to Open Loop, I've seen it myself with my scanner. The event happens when the coolant temperature drops below 45 degrees Celsius. Typically this will happen if you throw it in park after just barely warming it up then go and try to tune the carb or something like that. There is nothing about the way the O2 sensor is reacting that will cause the loop to change from Closed to Open in my review of the ECM code.

Agreed on the Rich/Lean trouble codes - if the O2 sensor sticks on either a very low or very high voltage for a concerning amount of time then trouble code 45/44 are thrown. Parameters are in the PROM here;

31B4:   2E                          FCB     $2E ;	MAX O2 CODE 44
31B5:   19                          FCB     $19 ;	MIN TIME CODE 44
31B6:   3C                          FCB     $3C ;	MIN TIME CODE 45
31B7:   AD                          FCB     $AD ;	MIN O2 CODE 45

Translating that: 25 seconds of less than 18% of whatever the maximum voltage is for the O2 sensor triggers Code 44 ($19 = 25 seconds, $2E = 46, out of 255 maximum that's about 18%). 60 seconds of more than 68% of whatever the maximum voltage is for the O2 sensor triggers code 45 ($3C = 60 seconds, $AD = 173, out of 255 maximum that's about 68%).

My view continues to be - A heated O2 sensor will only help the 4th Gen Monte Carlo SS if some PROM parameters are tweaked to change the coolant temperature thresholds. I watched it tonight, at 52 degrees celsius, ECM switched to Closed Loop.

Posted By: Bitflipper

Re: Strange RPO codes??? - 02/03/16 04:31 PM

Quick minor correction to my post - 8 kilobytes of code not 2.

One further thought - this question arose as a part of discussing adding an O2 sensor to a set of headers that did not have a factory installed bung. Depending on where one is able to retrofit a bung, there is merit to discussing whether or not the chosen location will be suitable to heat a non-heated sensor sufficiently to get accurate readings. That is a topic I have no opinion on. The ECM will drop to Closed Loop at the same time based on coolant temperature regardless of the O2 sensor; the O2 sensor does need to be hot enough at that point to send accurate readings to the ECM or the air/fuel mix will be out of whack.

I'm running the old emissions legal Edelbrock 6879 headers on my '87 SS with a standard O2 sensor installed in the factory provided bung. It bounces around rich/lean as one would expect so long as my carb is well tuned.
Posted By: mmc427ss

Re: Strange RPO codes??? - 02/03/16 09:08 PM

You could try getting the engine to operating temp, closed loop at idle, then disconnect the O2 while the engine is running and see if the ECM goes to open loop. Of course within a short period of time the ECM should throw a CE light and a code 44, maybe a 45.

Does the 079 ECM learn parameter for a sensor like the later non-OBDII cars do. On those cars when you replace a sensor the ECM needs to relearn the parameters of the new sensor. For instance a new MAP sensor may read 4.25 vdc at idle, the old sensor may have been 3.5 v. The ECM needs to relearn the idle voltage which is now 4.25. Our cars have VAC sensors, their output would be opposite of the MAP sensor, low voltage at idle, high at WOT, just to clarify.

I had a set of the TES also installed on my L69, high flow Random Tech cat and 2 1/2" mandrel stainless cat-back. On the TES the O2 is located pretty close to the head, still in the engine compartment. On full length headers the O2 could be 30" downstream and under the car floorpans. One of these days I'll have to shoot some exhaust pipe temps with the pyrometer to see how much it changes as you move away from the exhaust port. I have used that meter to check individual temps of each cylinder as it exits into the primary tube. Engine dyno headers will usually have a thermocouple in each pipe to monitor each cylinder exhaust temp. You can use that info to determine if a cylinder is rich or lean.

Currently running Hooker 2050 emissions headers on the new engine, again smog police in Pa necessitated that. I did have to move the O2 bung from the left collector to the right collector, this due to using mechanical clutch linkage, the Z-bar would just touch the O2.
Posted By: Bitflipper

Re: Strange RPO codes??? - 02/03/16 10:47 PM

The ECM will go into Open Loop eventually if you do that, but only because it's sitting there parked/idling and the coolant temperature drops below 45 degrees Celsius. I cannot see how a single voltage signal over one wire from the O2 sensor to the ECM is possibly going to advise the ECM to go into Open Loop even if there is (say) such code in that ECM that I'm not noticing.

Good question about the ECM and learning. It does NOT learn anything. There is not a single byte of Non-volatile memory in it. The sensors must be calibrated appropriately; for example, the TPS must be about half a volt with ignition on/engine off or you'll have all sorts of issues near or at wide open throttle (not to mention TCC lockup, but that's not ECM controlled so I digress).

The ECM barely keeps any memory of the sensor readings between its loop cycles even. There are a couple of bytes for knock counts and related timing retard for the past couple of cycles, two previous MAP sensor readings, two previous VSS readings, four previous TPS readings, not much more than that. It's a pretty simple computer and is operating for the most part on current state readings.

The ECM does remember a few things between starts. There is a counter for how many times the car has been started since a trouble code was thrown; it discards trouble codes after it hasn't seen one for a certain number of starts (IIRC it's 20 but I'd have to check that again). There is a hot restart timer that kicks it into closed loop quickly after a momentary stop/restart. Maybe a couple of other things, not much. Everything is "forgotten" if the ECM is unplugged or you pull a battery cable.

As for how far downstream an unheated O2 sensor can be mounted, that's where I'm way out of my league. I totally agree that it's possible for the ECM to decide it's closed loop time from the coolant temperature and run time but the O2 sensor is mounted somewhere that it's not getting hot enough to respond with a proper reading.

Re: Strange RPO codes??? - 02/04/16 11:00 PM

ok so I was straitening out some of the underhood wiring today... I cant find the wire for the o2 sensor or the power steering pressure switch? anyone have any idea what harness they come from? I don't see any cut wires or the pig tails for either? they were gone before I got the car so not sure where they would run originally...
Posted By: mmc427ss

Re: Strange RPO codes??? - 02/05/16 12:47 AM

The wire for the O2 is PURPLE, runs from the O2, up over the valve cover at #3 and into the CCC harness in front of the carb, lots of other CCC wires merge right there.

The switch wires on the P/S box pressure line is DK GREEN, that wire comes from the low pressure switch for the A/C, on the other side of the switch is a DK GREEN/WHT, it run to the A/C compressor.
That switch goes open when it reaches a higher than normal pressure and turn off the compressor. Such as when at idle, moving very slowly, and the steering is turned against the internal stops in the steering box. We have all heard our grandmothers do that, we don't do that, that's abuse.
I wouldn't be concerned about hooking that switch back up unless the wires are just laying there. I believe you will find the switch wires somewhere up in front of the carb too, most like follow the coolant temp gauge sender wire and follow it back. If the A/C works, compressor runs, then you have power coming to the compressor from the low pressure cycling switch. Someone has already eliminated the P/S switch from the circuit and spliced up in front of the carb.

A note, a primary CCC system ground point is at the front of the intake manifold, insure it quality.

Re: Strange RPO codes??? - 02/12/16 02:01 AM

Thanks i finally found it!
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