My car Is an '87 Z28; The previous owner transplanted the original 305 with a Powertrain Performance Products 4bbl 350 ci engine from the 87-88 Caprice ("LM1" engine I believe; exact link to the particular engine: http://www.goppt.com/ppt-web/catalog...ubmitting=true ). For what its worth, I've noticed that the car appears to have an Edelbrock intake, and the Rochester carb is numbered "17086004", indicating its from a 1986 car.

When I first acquired the car, The engine had a pretty severe miss that happened fairly frequently, but was still intermittent. Whenever the car would start to miss, it would throw an engine light. After replacing everything from plugs to plug wires to fuel filters etc., I took it to a shop where the guy was able to figure out that the distributor was actually hitting the inside of the cap (he said there were red plastic shavings visible) which was "causing the computer to act up & throw faulty codes".

After he fixed it, the car ran fine for a little while, but it began to act odd again. The car has an intermittent miss/stutter again, but its far less frequent, not quite as severe (most of the time), but most unusually, no engine light when it happens. I know the engine light works, and since the stutter has begun, I had the carb rebuilt and an exhaust leak fixed in order to pass emissions; this seemed to have no effect on the intermittent misfiring. Even more odd, is the miss seems to follow a fairly unusual pattern:

In the hot, 90+ degree California summer sun, the car will start up and run perfectly. If it is even remotely cooler (like 60s-80s; i.e. early in the morning, later in the afternoon), the car will start up fine, but the miss appears when the car reaches operating temp (about the 220; the center of the temp gauge).

So basically, when I start the car to go to work, it runs fine until I'm about half way there, I get a misfire the rest of the way there. When I start the car again for lunch, it runs perfectly, then when i go home, every other day or so, I will get a very very slight misfire for a few seconds every once in a while as I'm driving.

Any advice would be appreciated; I know this was long winded but I figured I should be thorough!

Thanks again!

 

Additional Info:

Varying fuel type seems to have no effect on the frequency or severity of the stutter. The baseline 350ci I have is designed for 87 octane but I've run 89 and 91 in it from different gas stations with little effect on the performance of the car. When the car does its misfire, its not consistent; It will miss for like a minute, then clear up, then miss again etc. This indicates to me that its something electronic, but I don't know why it wouldn't be throwing a code anymore. Also, right after the guy "fixed" the original misfire, he said my computer was still good, but who knows if it still is.

This is driving me nuts, any help would be greatly appreciated!

 

Having been there and dealt with it trying to replace parts all over my distributor to figure it out... I'd just replace the distributor. See if you can borrow a dist from someone.

I think mine ended up being the Ignition module, but it was really ambiguous, sometimes it would be there and sometimes it wouldn't.

 

I could have sworn he replaced the distributor, so I went back and looked at the invoice for when the car was repaired for this issue the first time, & I was in fact charged $102 for a "Ignition Dist. Module". As I mentioned, this "fixed" the engine light coming on but the misfire still comes up occasionally.

Is there something else that could be "causing" an ignition module or similar problem, if that is in fact it?

 

Could be pickup coil, that connects to the module.

 

note that for me, I went through TWO Accel modules. Misfire happened the whole time, the modules would fail after 3-6 months. Didnt really connect everything until I had replaced the second accel module with a borg warner module. The misfire would be intermittent, but it seems like around the times the modules croaked it would be worse for those few weeks. But when you're dealing with intermittent problems at 4k+ RPMs.... I dont like screaming down county roads at 5k RPM inviting law enforcement attention...

I THINK that's what fixed it. but I replaced so many ignition parts trying to find that misfire that Im surprised my hair didnt go gray. I was getting really frustrated with it. But I wanted to point out that I replaced the ign module, the car ran fine again, but it still had an intermittent misfire. I replaced that one when it failed and it seemed to fix it.

 

Hmmm, Thanks for the input, I will have to look into that. However, my car started acting up within a very short period of time of the distributor being replaced, and it doesn't appear to be getting progressively any better or worse, though I suppose it is entirely plausible that there is still a faulty component.

However, what relevance do you make of the car responding to barometric conditions? I started the car up today while it was about 85 and clear outside, and It appeared to have no problem. Just the day before yesterday, it was in the 60's & overcast & the intermittent misfire was rather severe.

Do you think there is any correlation, like perhaps the computer is intermittently messing with some kind of settings in response to atmospheric conditions, or do you suppose the temperature observation is irrelevant?

 

Hey there,

I've got an '85 T/A with the 305ci and factory rochester 4bbl, all factory and i'm experiencing the exact same problems. Literally identical to those you are describing. I must add though however that on top of all the barometric shenanigans, the problem seems to intensify when elevation change occurs. Where I live I sit at just over 60 feet above sea level and when I travel to work (3000 feet asl) it intensifies and won't return to I guess what you could call its 'normal misfire rate' until after I have driven at lower altitudes for a few days...

Any helpful info would be greatly appreciated. Been dealing with the annoyance for just of 6 months now.

Austin

 

So its been a while and I have been out of the state so the car sat for some time, but I'm back and looking to solve this aggravating problem once and for all. After searching these forums, these cars seem to have misfire issues but I have not seen any real solid or consistent solutions. I've driven the car a handful of miles since I've been back and the misfire is as random as ever. I drove on the freeway when I first got back, and for 25 miles the car ran fine, then the cooling fans stopped turning on (a whole other issue?!), the car got hot, and misfired after that. A few days later, I was driving and over the course of maybe 10 miles, the further i went, the worse the misfire seemed to get.

This intermittent misfire is driving me nuts. I took a step back and reviewed all that had been done so far. It looks like the previous owner replaced the ignition coil (i found a receipt), but I am not sure how long ago (milage wise).

I did some investigating and the "Ignition Dist Module" that was replaced was the Ignition Control Module located under the cap. I replaced the plugs and wires when i got the car (maybe a couple hundred miles ago tops?) , and earlier tonight I replaced the pickup coil, the cap and the rotor.

I was about to celebrate because the car ran smoother than I had ever remembered... for about 5 minutes. Car got up to operating temp (fan comes on again magically) then it misfired really bad. It shut off when I pulled into my driveway and was hard to restart and idled terribly. Let the car cool off a lil while, and it restarted, but my favorite misfire was right there.

I figure i may try re-replacing the ignition coil because thats the last ignition component i can think of, but if anyone has any other possibilities im open for suggestions. I am largely convinced this is an electronic issues because of the sudden way the problem appears and disappears, but I am desperately seeking any knowledgable advice.

Sorry for the long post again, and thanks in advance for any insight!

 

@ InfernalVortex, not even sure you even still monitor this old thread, but do you think it would be worth it to re-replace any of the previously replaced components? when you said you went through two accel modules, were those Ignition control modules (under the cap), or actual ignition coils?

 

I replaced the coil and it had no effect prior to replacing the module a second time. I only replaced the second Accel module because it failed and stranded me on the side of the road. Just went with a cheapie borg warner module and problem was gone. It has stayed gone too. Thats why I avoid "performance" ignition modules these days even though I have an fancy accel coil.

Anyway as to whether rplacing it again would help... hell I dont know. It worked for me. If you're pretty sure you dont have a lean or rich misfire and it's actually ignition related I would spend the $25 on a new module, and if that didnt make a difference I'd just keep it for a spare.

And remember, I have a carbureted car with an old school large cap HEI distributor. So I neverh ad to worry about the computer causing issues.

 

I have seen this type of problem before , and here's another thing to consider ;

Since you have replaced just about every ignition system component at least once , and still the odd miss remains , could yours be a problem of not enough current getting to the distributor to run correctly ? If you are at all handy with a voltmeter it would be a good check to monitor the 12V going from the ignition switch to the distributor ( usually the large pink wire ) .

I have seen several cases of "ran ok cold , misses when hot" end up being defective ignition switch contacts that make ok enough connection when cold but can't pass the needed current when warm .....

 

Also check for sparkplug wire spark leakage. at night in complete darkness pop the hood and mist the wires with a waterbottle and squirter/mister like a old windex bottle. If you have any arcing you have a bad plug/coil wire.

 

have you changed the temp sensor for the ecm and verified all wiring/grounds are in tact? computer controlled vehicles reference temp for fuel/spark.

 

Thanks guys for the good suggestions; I'm going to pull out the multimeter and chase wires today and check out my grounds and hot connections. I actually chased all the grounds I saw and cleaned up some of the connections a week or two ago. However, when I look at the quality of the connections made and wiring done when the previous owner put this motor in, it is not outside the realm of possibility that bad connections persist.

I have not changed or specifically looked at the temp sensor yet for the ECM; I suppose until yesterday I didn't see any specific correlation with temperature as the car would miss randomly from being completely cold to hot, but I will look into it.

As for the spark plug leakage, I had not thought about that as I assumed they would still be good considering both the wires and plugs have a couple hundred miles tops, but that may be worth checking out as well in order to be thorough.

 

temp sensor misfire? i think i need to grease my muffler bearings...

what brand distributor was used for replacement?
I do fleet parts for a municipality and have seen many A1 Cardone and import (Dorman cough cough) failures.
Verify correct TPS voltage (~.5V at idle with a smooth read up to 4+V at WOT), could cause a stumble on tip in if out of adjustment or going wacky. CC Computer is too stupid to determine a faulty TPS and set a code.

My L69 also had a faint rhythmic misfire/surge I was chasing down for awhile.
I rebuilt my OE dizzy myself. Turned out to be a deteriorating pickup coil-the windings were physically breaking down and the magnet had hairline cracks. Also put in a fresh US made BW ignition module with heat sink compound. MSD cap and rotor and wires. Verified coil resistance and endplay. Misfire solved. Before the rebuild the car would also start to break up around 5k rpms, now it purrs all the way to valve float

don't trust parts just because they are "new".

 

Some one always needs a smart *** remark. Temp can affect fuel and spark tables in the ecm.

 

Some rather unusual stuff going on electrically. So I got around to checking the voltage of the TPS sensor on the carb.. Correct me if i am wrong, but the top plug should have 5 volts, the middle is the signal that ranges from .54v to 4.0+, and the bottom is a ground right? I checked and rechecked and rechecked again and there was no voltage at the middle prong, regardless of where i moved the throttle linkage. there was no voltage at the top prong either, and just for giggles i checked and there was none at the bottom. I checked the bottom plug for a ground and got a weird reading where all the digits on my multimeter just went out..

checked the top and middle prongs to see if they were grounded and they were not.

But the thing that makes this really weird was that i decided to check the plug that goes INTO the carb plug (from the wiring harness) - each of the three prongs were at the exact same 8.51 volts? Is there supposed to be any power going TO the TPS???

To be thorough I confirmed that my multimeter was working on a known hot wire, and noted that the battery was at 11.75 volts (admittedly low for some reason)

Anyone have any clue whats going on here? do i have some sort of crazy short somewhere? im not quite sure what kind of short would be giving readings like this

 

my car is not throwing any check lights btw, but ever since the other day after i changed the pickup coil and it ran great for 5 minutes then terribly afterward, I havnt really been able to get it started. It will kick over once then die, with a final shot of fuel just smoking in the carb. with very much effort, it sort of "runs" if you keep throttle in, but its runs as if the timing is WAYY off or something (i confirmed and the distributer didn't rotate since i reinstalled it), and that car backfires and misses like crazy. I still feel like it is something electrical because it literally started like a light switch was turned on.

maybe a sensor or something is giving the ecm crazy values?

this issue seems to get more aggravating by the day..

 

TPS MUST be functioning properly in the CCC system. Check your grounds and wiring to TPS for frays and breaks.
Key on when you checked voltage?
This needs to be worked out for the CCC system to work.
TPS gets 5v like all other sensors.

Im am starting to lean more towards carb issue if you have accurately checked your ignition system. ECM is also in the back of my mind because that is where the TPS gets its power from...

 

The key was in fact on when i checked. I traced the three (grey, blue, & black) wires going to the TPS to where they meet the main bundle and didn't see any damage; I will attempt to perform a comprehensive check shortly and trace them back as far as I can.

Again, correct me if I am missing something or interpreting it incorrectly, but from looking at the wiring diagram, these three wires come straight off of different pins on the ECM, so for them to all be supplied with the same (8.51v) voltage means something powered is either shorting all three wires at the same time, or the ECM is supplying this voltage (to all the wires no less)

I am wondering as a bit of an aside- it is my understanding that there should be a ground strap from the passenger side head to the firewall, but the only firewall ground strap i can manage to find actually goes to this bent bracket looking thing mounted on the intake (i think its an intake bolt- i am going from memory). my question is where does the ECM get its ground (location wise)? does it have a ground to anywhere in the engine bay?

 

You are correct. Copper ground strap from pass head to firewall.
There are 2 ground wire rings bolted directly to the back of the pass cyl head PLUS the ground strap to the firewall. I also believe there is a wire that grounds to the dizzy.

Do your other sensors receive a 5V signal? Carb ECM's are really inexpensive used.

 

Update: It seems like every time i go down one path another issue compounds the problem. I managed to clean up a few wires and discovered that there are several instances where the plastic tabs that hold various connectors tight are broken off. Its like the previous owner broke most of them when putting this motor in. However I am not convinced any of these connections are bad enough to cause such sudden misfiring (correct me if I am wrong though) because it doesn't appear that any of them are coming loose.

In any case, while chasing wires I got around to checking my ignition coil and it appeared that it was bad so i replaced it (so as of now: new pickup coil, new ignition col, new(ish) plugs and wires, new cap & rotor, new(ish) ignition module)

The car still seems to kick once immediately and then die. To add to the issue, my starter is doing something strange suddenly. I posted a link to the video because that describes it best, but its like the starter (or something) is staying engaged and grinding the motor to a stop.

http://www.youtube.com/watch?v=hucMJ...ature=youtu.be

Its like seriously wtf is going on with this thing. Is this just a simple case of a bad starter, or could more wiring gremlins be keeping the bendix engaged? (as a side note, I didnt think a starter bendix would have enough resistance to grind the motor to a stop, but apparently... needless to say, I am not really starting the car until I get this issue figured out to avoid any further damage. I looked at the flex plate and it doesn't appear that any teeth are missing or ground off yet.)

like usual, any insight is appreciated with my basket case Camaro..

 

as a side note, I have not yet confirmed if all the other sensors are getting 5v yet, nor have i figured out why the TPS is getting 8.5v from all three wires coming from the ecu. I keep looking and cannot seem to locate any shorted out wires or grounds disconnected. I did disconnect one ground wire ring that had two wires running to it and noticed that even with it disconnected, it was still grounded (multimeter showed continuity with a chassis ground) does that sound right? I figured a wire that was already grounded wouldn't be grounded again somewhere else

 

So the starter issue was just that- a bad starter. After being replaced, the Basketcase Royale still refuses to fire. It will sort of "sputter" only while being cranked over but never actually starts. It appears to be getting fuel so I'm not sure thats the issue. As i mentioned in the previous posts, both the ignition and pickup coils are new. I pulled all the plugs and inspected and cleaned them up before reinstalling. they were al lil black but didn't seems too far off normal.

Some of the other posts i've been reading seem to mention adjusting the IAC, but most of the information seems to pertain to TBI and TPI cars.

I briefly considered that perhaps the timing was off, but the car ran perfectly with the timing exactly as it is (with regards to distributor location) so I dont know if it would suddenly be so far off the car wont start.

Any suggestions to at least get the car to fire so I can continue to diagnose the issue? (the misfire that originally started all this lol)

 

So the issue has turned a new leaf and I appear to have made some progress. The car not starting seemed like it might have been as simple as a fuel delivery issue. A little starting fluid and some coercing got the car to fire last week and the strangest thing happened: it ran lol. The last thing I replaced was the ignition coil and it is possible (fingers crossed) that this has resolved the major stumble. I have been watching it the last week or so and I have not had any running issues yet. I had to get it smogged last weekend and my timing was just a tiny bit off, but after some minor adjustments the car is still running fine.

The idle seems just the slightest bit high once its warmed up at a little above 1000 rpm but that could be anything from vacuum leak to it just being adjusted off. I will update if the situation progresses for the better or worse. Knowing my car I'd be amazed if this was the end of it.

As a side note, I never followed up with the issue with the TPS, yet the car still seems to be behaving. I figure I'll watch how the car behaves some more before prodding any further..