Midnight 85

Guys, I have a problem I cannot figure out. I will describe what I am running first, "85 Vette with upgraded ecm. Edelbrock Pro Flo XT manifold w/ LS style throttle body. 383 engine dynoed at 435 hp @ the flywheel.
Now the problem, I can drive the car at a steady speed and when it gets fully warmed up it stumbles like the key had been shut off for a split second, yet I can floor it and it will run like a scalded dog. Once it acts up for a while it will run okay again for another 4-5 minutes then it starts again. It seems to be heat related.
This is what I changed so far, most of this stuff I had laying around so very little money spent.
1. ECM, Converted to 165 years ago.
2. Ignition module, ( small cap hei)
3 Cap,coil, rotor
4. TPS.
I am starting suspect the distributor itself but don't think that is it.

mlm0

Can you hook up fuel pressure gauge to see if pressure stay steady during times of problem

antfarmer2

Did you use the right thermal grease on the ICM?

Midnight 85

I have a fuel pressure gauge in a pod on the A pillar, it never so much as moves when the car stumbles.

Midnight 85

Yes I did but I am wondering if the module might be defective. Could it be the distributor itself?

calvinlc

I had a similar head scratcher problem a couple of years ago. The car would not start sometimes and sometimes it would just start running rough for no apparent reason. When it was running it was running fine. I replaced the coil, ignition module, etc. - this was a large cap distro. I eventually got fed up with it, knew it was electrical, and just bought a new Jegs billet small cap distro and converted and it has never stumbled since. I suspect it was the pickup coil in the distributor - not the ignition coil, but pickup. I understand how furstrating it can be. Have you verified if the fuel mixture is changing when the condition happens? In other words, could it be an injector driver bank or something like that? How about a loose connection the ignition to ECM area? I hate these types of problems.

Midnight 85

Fuel mixture is dead on and I monitor it with an A?F gauge mounted on the A pillar right above the fuel pressure gauge. I'm thinking it may be the stator too and they aren't expensive so I may get one tomorrow if I can find one on Sunday. Thanks!

blackozvet

I had a similiar issue, it would have a random stall, turned out to be a throttle body vacuum leak.

Midnight 85

This isn't really a stall, it is more of a stumble and always at highway speeds but thanks for the suggestion.

Midnight 85

I may have found something, I pulled the dist. so I could change the stator. The shaft has a lot of play in it up & down. I'm going to check that with a feeler gauge later today. The new rotor looks to have had some pressure put on it already and I just put it in. This dist. was brand new, maybe 5000 miles on it.

calvinlc

This is the EXACT sequence I went down I tried shimming the distributor with no luck. Not saying you won't see different results but.....keep in mind, I think it is also harder to find shims for the small cap distributor as the shaft size is different than the normal HEI if I remember correctly. The other note is that a lot of the factory cars of the 70's and early 80's routinely ran HEIs with 45 to 60 thousandths end play.

cardo0

A data scan that captures the behavior should help. It reads like u have enough $$ invested that scanning software and equipment is the next step. It's just that scanning gives u a look at fueling and timing and all the other device status while the problem occurs.

Good luck.

Midnight 85

Mine has 69 thousands end play, on top of that the gear was real worn on the mesh side of the teeth and looking down at the stator the shaft was offset to one side. I decided it would make a great pre-oiler, and I ordered a new dist.

My next step was going to be a scan but with what I found when I pulled the distributor, I'd bet my source of aggravation is never going to bother me again. This is what I get for using a cheaper reman.
Thanks to all of you who helped, I will post results when I get the new distributor installed.

calvinlc

Since yours is the small cap this probably won't be an issue since I think they all had them, but make sure your new distributor has the melonized, or hardened, gear as opposed to the standard cast iron one that most of the big cap HEIs come with. Otherwise the results won't be good when meshed with your roller cam - assuming you have the standard hydraulic roller setup.

Midnight 85

Yes it is a hydraulic roller and the new dist. has the steel gear. Now that you mention that, I'm not sure the old one had a steel gear, I guess I better check the cam gear just to be sure. I can't believe I overlooked that. Thanks again, you've been a huge help.

antfarmer2

How old is your o2 and did you change it to a heated one if you meesed with the exhaust?

Matatk

If you are running an aftermarket cam with a steel gear (not a pressed on) along with a steel or melonized distributor drive gear, you're going to chew it up. You need to get the composite gear from Comp Cams.

I chewed up two melonized gears before I switched. No problems now with over 10,000 miles on the composite gear

http://www.enginebuildermag.com/2009...tributor-gear/

Midnight 85

O2 is only a year or so old, less than 2000 miles on it and it is heated. Thanks

Midnight 85

I bought the engine from ATK in Texas, who, by the way, build one hell of an engine. They told me to use the steel dist. gear. I got out the warranty papers this morning just to be sure. Thanks.

Midnight 85

The new distributor took care of my problem, no more stumble. Thanks to everyone who offered advice it is greatly appreciated and a special thanks to calvinlc, he nailed it right off. The defective dist. was a Delco reman. I now have a Davis dist. and it is a good looking piece, and, brand new.