JACKRBT

Disclosure, this is in an '87 Corvette but I'm hoping you guys can help.

Last fall, the car was running great, no issues. Over the winter, I did some preventative maintenance and replaced plugs, wires, cap, and rotor. I also replaced the AIR pump with a rebuilt unit since the bearings were starting to rattle in the old one.

Anyway, firing it up this spring it starts great, idles fine, but there is a miss that is happening under very light-to-no-load conditions when the RPM gets to about 2K. I can get it to happen just revving up to that range in the garage. When driving, it happens regardless of gear (auto) but only under steady cruising with very low load. Pulls strong, good power. Even going up a slight grade, it seems fine. As soon as the grade levels off, it starts happening again. In short, it is very dependent on load/RPM rather than throttle position.

Since the coil was not replaced, I changed that with a new one but it didn't make any difference. The engine is stock and the parts are all new OEM-spec AC Delco.

I have the factory manuals but this problem doesn't fall neatly into any of the symptoms. I thought maybe I had inadvertently disturbed a vacuum line or fitting somewhere but I can't find anything out of place. I'm planning to check the resistance of the wires in case one or more may be bad. Plugs were all gapped correctly.

I did notice soot around the center electrode of the rotor after pulling things apart again. I don't remember seeing that on the old rotor and that thing was over 15 years and 30K miles old. I'll check resistance on the rotor too. Should that be virtually zero? Manual says wires should be 30K ohms or less.

I have access to an infrared thermometer so I had the thought of checking the exhaust manifold at each cylinder after running it for 20-30 seconds to see if there is a cooler cylinder that's not firing.

Any other thoughts or ideas? Thanks!

jamon8

iTrader: (3)

someone could have dropped one of your plugs

JACKRBT

Since I'm the one who did the work, that someone would have to be me. But I didn't drop anything...I swear!

Es_91rs

Did you double check the gap on all your plugs and make sure all your plug wires are seated properly? The C4 vettes can be a pain to get them seated properly, especially on the passenger side. My dad has always had a vette and I've worked on a couple of the C4s he's had.

JACKRBT

I did check the wires but I'll do that again. I will eventually get to pulling the plugs and checking them again too. Hoping I find the issue before that because, as you say, the passenger side is a major knuckle buster!

JACKRBT

I finally got around to working on the car again. Checked the resistance on all the plug wires. The highest was 20k ohms and those were the longer ones. Pulled all the plugs and they looked fine. Gaps are correct at .035. All wire connections are secure at the plugs and the cap. Problem is still there; still no codes. The thing runs absolutely great outside of that narrow band of parameters. I'm really stumped on this one. Getting close to taking it to a professional but that always makes me cringe a bit.

EDGE

Have you looked at your ignition module underneath the cap? That thing can cause real odd behavior up to and including a complete shutdown. I had two fail on me, one would not allow me to get above 3000 RPM. When I did it backfired so hard that it blew out the right header gasket and hollowed out the catalytic. The other one started to run rough, eventually spitting and sputtering, then it would finally stop running altogether. This is the solid state device screwed to the distributor base. The cap has to be removed. It's what gives your tach signal and the initial charge to the pick-up coil. Not very expensive, but when it goes it's a real nuisance.

JACKRBT

I've wondered about the module. Seems there is no way to DYI test those. Given the modest cost, I ordered one and will try that. Thanks!

aliceempire

iTrader: (14)

If you have a good parts store near you, they can typically test it. I wouldn't just throw a part at a car if I can test it.

EDGE

Mine was fine cold, for about 2 minutes. When it acted up, I tossed it in the freezer for twenty minutes. Put it back in the car, and again it was fine for 2 minutes.

86LG4Bird

iTrader: (1)

JACK, sorry I just now saw this thread.
I've formed my own theory/solutions for this issue based on years of observing/fiddling with them. The key things you mention are: replaced cap, and soot on the rotor.
Are the cap & rotor cheap auto parts store brand? I've found time and time again that an OEM-quality 150k cap & rotor are better to leave in than cheap replacements. The cheap plastics are very prone to absorbing moisture, providing electrical leak paths inside the cap. Pull the cap, dry it thoroughly with a heat gun, and I bet your symptoms go away (temporarily, that is!)
You'll notice the symptoms at above idle / low load / light throttle conditions because that's when ignition timing is most advanced and the gap between rotor and the outer post inside the cap is at its greatest. It's easier for the current to take the cheapo-plastic-moisture-ridden leak path than it is to jump the gap between post and rotor.


You've already ruled out other ignition components since it functions fine under max cylinder pressure conditions (WOT at your torque peak).


Now here's the bad part: It's getting harder and harder to find OEM-quality caps for these old cars. What I've done with success on a few vehicles is to semi-seal the base/edge of the cap with grease when installing it, and also dab some grease under the vent holes in the base of the distributor. This will work for quite a while until the ozone contaminants build up and THAT debris inside the cap will start causing misfires. But it seems to take 20-30-k miles before I see that happen.
For a long-term fix, do the semi-seal grease thing, but tap into one of the base vent holes with a vacuum fitting routed/teed into the brake booster vacuum line. Be very sparing with your grease, and you won't suck any into the distributor. Once it has one tiny leak path, it won't pull anymore in.
I know people have sealed distributors with silicon/RTV, but for me the grease option leaves a lot more flexibility to still disassemble when needed, and it seems to do the job just as well. The cap-to-base fit is very precise, so it takes only a thin film on the narrow bottom edge of the cap.


Do you still have your old cap? Try re-installing it.

JACKRBT

Thanks for the reply. All of the parts I used are OEM-spec AC Delco pieces. Absolutely no off-brand parts store stuff here. Unfortunately, when I was doing the majority of this work in February, I tossed the old parts. Since then, the car has been out of the garage, in completely dry weather, a total of maybe 1 hour. I've had the cap off many times chasing this thing and have found no evidence of moisture.

I did think the soot on the rotor was questionable because I don't remember seeing any evidence of that on the old part that had been in there for over a decade. The use on these new parts can be measured in minutes! Others have said the soot is totally normal. Anyway, the cap and rotor are coming back out soon because I've got a new ignition module on the way. I'll sure be taking a close look at all those parts again. With the weather finally getting nice (well, not this week), this is really getting irritating.

JACKRBT

If the new module doesn't have any effect, I'm going to seriously consider trying a new rotor. I've put a meter on it and all seems good from what I can tell. But once it gets hot, who knows? It's about the cheapest part to swap!

86LG4Bird

iTrader: (1)

The cap is more problematic than the rotor. Current has an easy metal path on the rotor; where it needs to arc to the CORRECT pin in the cap is where the problem arises.

JACKRBT

New module had no effect. Problem still occurs. At least in park while revving in garage...crappy weather today so I'm not driving it but it behaves the same in the garage as it did before. I can't say 100% that it's tied to open vs. closed loop operation but it does seem to only happen after it's warmed up a bit. About when the idle drops, which I guess is a good closed loop indication. I need to go get a scanner and see what it's doing I suppose. It also seems like it has a slight miss at idle too. It's just not as pronounced as when it gets close to the 1800-2000 rpm range.

86LG4Bird

iTrader: (1)

Yep, closed loop would be when the ECM bumps up the timing in that low load, medium rpm cruise condition, giving the spark currrent inside the cap it's toughest path.

JACKRBT

I've noticed that there are two different AC Delco rotors listed for this engine (D468 and D448X). The D448X is what came in my "tune up kit" from MidAmerica and is white in color with phillips head attachment screws. The D468 is black and has hex screws. Beyond that, the D448X is about $5 vs. around $13 for the D468. Is there a functional difference between these two? I know the rotor I took out of the car had hex screws and was blue in color. I'm tempted to get the D468 and see if that makes any difference. I've searched the interwebs trying to find any comparisons but didn't have much luck.

86LG4Bird

iTrader: (1)

Yeah, that MidAmerica $5 part doesn't sound like "All of the parts I used are OEM-spec AC Delco pieces"
That being said, even with the cheap caps, I've been able to make the problem go away TEMPORARILY by drying out the cap with some heat and reinstalling it. But the first time the vehicle sat more than overnight in warm and/or humid conditions, the problem returned.
The last good cap I bought that worked well was an ACDelco from rockauto.com It was $30-something.

JACKRBT

They are all AC Delco parts and they each came in their own AC Delco box. AC Delco's own parts catalog also lists both of those rotors for this car/engine. MidAmerica may have chosen the cheaper of the two for their kits. The kit was just more economical than a la carte for the parts I wanted. Only one AC Delco cap is listed so I'm assuming that is the cap I have (since it came in an AC Delco box). I'm going to try the other rotor and see what happens.