86REDC4

I have a modified L98 engine. It starts fine, runs fine, and idles fine cold, until it warms up and then the idle gets rough and is not stable (lopping) and may occaisionally stall. I wanted to check the timing, and I put a light on it and can't see the mark even though it's been painted. I found the 4 wire connector with the EST from the distributor that has the tan/black wire. But the wiring goes behind the engine to never never land and I can't find "set timing connector" to disconnect to set the timing. I've read several of the 86 rough idle threads. Any help would be appreciated.

MrWillys

Isn't the connector behind the power brake booster? Set base timing to 6 degrees. If you cam is bigger you may need custom programming.

86REDC4

I've looked and looked and can't find it. I have the 1986 Corvette shop manual and it doesn't show a "disconnect connector" after the 4 wire connector in the "component location section", and in the "ignition system check" chart C-4A drawing, it looks like they drew the connector in with a marker after the design was made.

Cliff Harris

This is your guy. Right next to the brake booster and under the windshield wiper motor:

86REDC4

Thanks MrWillys and Cliff Harris for the helpful information. The reason that I could not find the wire was that it was partially covered with electrical tape. But, I did find it. The picture was a big help. I will check the timing out soon, although I don't think that has anything to do with the rough idle problem. That problem seems to be something that happens when the engine gets hot and something is being switched on or off and a sensor or a device is malfunctioning (like the EGR). Thanks again guys.

Cliff Harris

I had your exact symptoms. It turned out to be the rubber hose that connects the PCV pipe to the intake manifold. What was happening was the rubber got soft when the engine was hot and got flexible enough to allow air to get sucked in. I only found it because it made a noise when it was hot. The original hose was glued to the PCV pipe but the new hose I put on there was not. I put a Ty-Rap on it and that fixed it.

86REDC4

I checked the hoses and they are all tight. The entire intake system was tested a couple of months ago as part of a dyno run shake down test. About 95% of the engine vacuum lines were replaced a couple of years ago.

hitmanpty

Like I all ways say. get back to basics.

first in your case I will check the fuel preasure. them I go for the timing. and just to make sure I will check the spark.

may be you have some clogged fuel injectors

hitmanpty

Like I all ways say. get back to basics.

first in your case I will check the fuel preasure. them I go for the timing. and just to make sure I will check the spark.

may be you have some clogged fuel injectors

86REDC4

The fuel pressure was checked and set at 43.5 PSI (it has a guage on the fuel rail and adjustable fuel regulator). I tried to check the timming a couple of days ago, but the mark did not show where it should have, even though it starts fine and runs O.K. until it gets hot and then the idle gets erratic or rough. I read somewhere that the harmonic balancer can shift or rotate, but I don't understand how that can happen if it's keyed to the crankshaft as many engines they are (I've not worked on Chevy 350 motors where I've disassembled them). The fuel injector are new (less than 500 miles). Thanks for the help.

86REDC4

One thing I did notice is the fuel pressure guage while running hot at idle the needle is vibrating with a high and low of maybe 8-10 lbs. swing and I averaged it. When the engine is off the guage reads a stable pressure with a single reading and eventually bleeds off over time. I don't remember if I had this reading problem when it was installed. And I don't know if and when the fuel filter was ever replaced, or where it is for that matter. I'll check it when it's cold to see if there's a difference, since at idle it's O.K. cold. I may haf another guage (the kind with a shrader valve disconnect).

jan-erik

Hi.
I have an 85 slightly modified with larger cam, more compression(not much) and ported top and lager runners and ported plenum.
Had problems with idle and stalling when stopped at red light
I made an min idle air adjustment. Put the min idle up to 650 instead of 450rpm as my idle is sat at 850 in the chip. I adjusted the IAC to a little opening when idleing so that a little movement give a larger change in the airflow through it when compensating for a possible stall and with using AC. Since this the car idles good with a lope ue to the cam <. <some times though there is a slight hunt, but not to worry about as the manual 4+3 tends to do that occasionally
Good luck

Cliff Harris

The harmonic balancer is made up of a hub with an outer ring. The outer ring is isolated from the hub by rubber. Sometimes the outer ring comes loose. It can walk into the timing chain cover (happened to me on my '86) or rotate so the engine can't be timed (happened to me on my '78 Corvette). Here's a pic so you can see the various parts:



HitmanPTY has a great picture of his on here. It was really junk. Here ya go. Check this out:

http://forums.corvetteforum.com/1580595003-post23.html

hitmanpty

My harmonic balancer was off the center just separate from the outer ring . And the mark shift. I did post Photo on that old post i did on the hb. I nil try to get you the lino later