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#1073425 03/10/22 05:42 PM
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1986 MCSS, stock CCC 305.

Sometimes it will run just fine, other times it will start missing very regularly, 2-3 times a second, accompanied by a quick flash of the CEL. If you rev it up, sometimes it will clear up for a little while and drive normal. When it comes and goes, it's like a light switch. I have replaced the cap, rotor, coil, plugs, wires, ignition module, and finally the distributor base with pickup coil. I found loose connections in the cap and a plug wire, but it hasn't helped the problem. The car sits a lot... maybe it's a lean miss from junk in the carb? It just seems weird to me that it comes and goes so abruptly if it's a fuel problem. Should I dig in to the carb or what? The only other thing I can think of it do is to hook up the WinALDL and see if I can make any sense of it, but I'm not very hopeful.

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I couldn't say for sure, sounds like you have been tracking down all of the typical stuff I'd think of.
If you are looking at carb stuff from sitting, I'd recommend doing a seafoam treatment on the air side and the fuel side both to rule out deposits that could cause the lean miss. If you haven't done it before, make sure you aren't in your garage and that the wind is blowing towards the neighbors you like the least, as it will make quite a smoke show, but it does a good job on all of the typical deposits a lot quicker than taking the carb off and washing parts down.
The fuel side is fairly easy, you can either run it high strength directly to your fuel bowl and crank it/pump it till it's gone (I haven't had much luck running an engine on anything more than maybe 1/4 sea foam and 3/4 gas), or go the low concentration route by adding some to the fuel tank. I'll typically do 2-3 times the recommended dosage if I have a problem I'm trying to fix, and use the standard dosage as more of a maintenance thing when I catch it on sale.
Air side treatment is where you make the smoke, just pour enough it at elevated idle to make it stumble but keep running. Keep that up for most of a can, and when you get to about 1/3 left, go ahead and stall it out, let it sit ~5 minutes while you reinstall the air cleaner, then go for a Spirited Drive to clear it out.

I've done it one probably 10 cars and never had a bad result from it, just some rough running while doing the treatment but running smooth ever after that. That might not rule out any fuel side issues period, but it could be a good first step.


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As you know, the OBD I is not conducive to saving random cel codes. Driving it more is certainly going to help. This might be just me, but I'd run some BG44k thru it first and give a good long run at highway speed. Good luck!!


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Fuel filter at inlet of carb, just get a new one put it in and try. I had some of the same symptoms and drove myself crazy changing stuff and it ended up being the filter. Worth a try.

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Misses driving or at idle? How many miles on it?

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Does it miss after the choke opens or closed loop?


SBC powered 1987 Regal with TES headers, ZZ4 intake, ZZ4 PROM chip, mini starter, THM2004R, 2500 stall converter, 2040 cam, CCC system, and 3.73 posi rear.

2008 ex NPS P71 Crown Victoria, cop motor, cop shocks, cop brakes, and Jmod.

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Thanks for the suggestions guys, hopefully I'll be able to try them out tomorrow if the weather isn't too terrible.

It misses at idle and while driving, cold or warm, and it comes and goes.

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Could be a loose or dirty connecter or ground.


SBC powered 1987 Regal with TES headers, ZZ4 intake, ZZ4 PROM chip, mini starter, THM2004R, 2500 stall converter, 2040 cam, CCC system, and 3.73 posi rear.

2008 ex NPS P71 Crown Victoria, cop motor, cop shocks, cop brakes, and Jmod.

Never argue with an idiot.
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Because the cel light comes on when the miss occurs,this sounds like a electrical problem to me. When the fault occurs, does it feel like a single cylinder miss or does it feel like the entire engine cuts out? On 1996 cars and up the cel light may flash when a miss occurs but doesn't on our cars. Typically a hard miss in a single cylinder is ignition (in the secondary side like plugs,wires,cap or rotor) and a mild miss is fuel related if that helps. If the entire engine cuts out, I would think that's a electrical problem. I hope this helps and good luck!

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Here are a couple videos from a couple weeks ago. I didn't get a chance to touch it last weekend unfortunately.

https://youtu.be/OM-n8AFFF1M
https://youtu.be/0y85xCD0G98

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Seems like electrical rather than fuel based on the video, but I'm still no expert. I'd probably start with an analog voltmeter or a test light and check power to the ignition system with it idling and cutting out. My guess if you have already replaced the whole lot would be that there is a blip happening in the power feeding the ignition system altogether that is cutting it out (or possibly in the computer system that is giving the spark advance signal?). Watching an analog meter would show those blips, then you would need to trace it back until you get steady voltage.
Not sure how to check the computer output, but I had a similar acting issue on a totally different setup (5.3 swap, cutting out under load) that ended up being a loose ground that fed the ECM and grounded the fuel injector circuit, and the "under load" part was actually when the throttle cable moved the bundle of wires, making the ground connection wiggle. I'm not saying that's your issue, but to me it seems like there is a total cut out that the computer "sees" instantly, like it might actually be seeing the same cause (loss of power/ground to a portion of the ECM) or has a direct line of communication to (something in the spark control?).

Last edited by Hunter79764; 03/16/22 09:16 PM.

Shawn

'85 MC with budget 5.3L swap, TH350 with stock 2.14 rear end
It ain't much off the line, but it's nice on the highway
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It's hard to tell from watching the video because you can't "feel" the miss. Sometimes the seat of your pants is the best for diag of a problem like this. At least it can give you a direction to go in. I would start by using a wire diagram and load test the power and ground circuits. I use a old fog light bulb,but you can also use a 12 volt test light or something like that to do the test . When checking a circuit,wiggle as much of the harness as possible and watch the bulb (and you can tap on things such as the fuse box). If it starts to flicker,trace the harness for the fault. Or you can go online and google how to do a voltage drop test and do that as well,but just using a load such as a test light would be easier. For something intermittent or random sometimes I've even put some inline bulbs (suck as marker bulbs) and watched the bulbs when the fault occurs. Like if you think the problem is the power lead at the distributor,put the test light in the back side of the terminal (don't jab the wire!) and see if the test light flickers when the fault happens. At least the car is stock,so a factory diagram can be used.

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I also had a problem with my car as it would fry the pcm (or ecu). I measured the resistance of all of the solenoids and found the egr solenoid was at I think 2 ohms while all of the other solenoids were at 15 ohms ( I don't remember for sure because that was around 5 years ago). It turns out that shorted solenoid was causing damage to my pcm. Maybe hook up a tester to the data link under the dash and see if it looses communication when the fault occurs. Or try to find a chevy dealer with a tech that worked on these under warranty and see what they think (I know that's a long shot,but maybe worth a try). I think the fault is something tied into the computer or harness somehow because of the light flashing. Sometimes with electrical the hardest part is knowing where to start. Good luck!

Last edited by zelm86ss; 03/17/22 01:06 AM.
zelm86ss #1075295 09/27/22 06:04 PM
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Originally Posted by zelm86ss
I also had a problem with my car as it would fry the pcm (or ecu). I measured the resistance of all of the solenoids and found the egr solenoid was at I think 2 ohms while all of the other solenoids were at 15 ohms ( I don't remember for sure because that was around 5 years ago). It turns out that shorted solenoid was causing damage to my pcm. Maybe hook up a tester to the data link under the dash and see if it looses communication when the fault occurs. Or try to find a chevy dealer with a tech that worked on these under warranty and see what they think (I know that's a long shot,but maybe worth a try). I think the fault is something tied into the computer or harness somehow because of the light flashing. Sometimes with electrical the hardest part is knowing where to start. Good luck!

You were right. I had tried your suggestion months ago, using WinALDL to connect to the ECM and I couldn't find any correlation between the miss and anything in the data. Each time I worked on it, I would find a wire here or there that didn't look great, but wasn't broken and I'd repair it. Well a couple weeks ago I tried WinALDL again and discovered that I had the baud rate set wrong. I had it set to 2400 which is what the help documentation recommended for that ECM. It would read data, but it was missing messages. I changed it to 4800 and I started reading periodic data. From then on, whenever the engine missed, it would stop streaming and I would be missing packets. I grabbed another ECM on Ebay and that seems to have done the trick. There are no codes and the sensor values seem reasonable. Re-reading this post, I'm a little worried that a solenoid might have caused the old one to act up so I guess I need to ohm them out.

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Good job finding that! Be sure to check the resistance as well so you don't burn up the new pcm (ecm).


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