jbert55

Hi, I am new to this forum, glad to find it!! I am a middle age guy that has gotten back into Camaros after a 25 year hiatus! Last one was a '67 convertible, so wanted to stay in that style, at least the convert part!

I have a '94 Z28 Convertible that I bought online, flew to LA, and drove it home! (1800+ miles) My car is a 2 owner, Grandson and Grandma, believe it or not! Great shape, 118K miles with good detailed records of maintenance.

I do have a problem that has cropped up in the last month that I can't figure out. I will be driving down the road, and the car just quits..no "burp" or stumble, just stops running. I can put it in neutral, try to start it as I'm coasting, and it just cranks and cranks. I pull over, sit for a minute or two, and it starts back up. This has happened -5- times in the last 15 outings, so the really bad part is it doesn't do it all the time!!
Took it to my mechanic, no codes so it is quite hard to diagnose. I will say that once in a while it cranks for 15-20 seconds before it fires, but not all the time.
I am wondering if it is fuel or spark, and how to troubleshoot it.
Any help would be greatly appreciated!!

Injuneer

"New Member Intro" is not the best place to post tech questions. I am going to copy this post to "LT1 Based Engine Tech" where you are more likely to get timely help.

After copying it, I will come back and edit your post to delete the tech question, and post a link to the new location. All that might take a while, so go to "LT1 Based Engine Tech" and look for the copy of your post. There are people here who can help, including me, so please be patient if you do not see an immediate response.

And..... welcome to CamaroZ28.com !

BMX Dad

If it just shuts off, a good place to start would be the ignition module. They are known for getting hot and then failing. After they cool off they tend to work again...

Mine did the same thing around 120K miles. Put cooling spacers on your ICM if you replace it.

Fatdog2

If it was were me I would first replace the gas filter.next I would check the cable coming from the distributor (optispark)to wherever it goes(Icm I think)for burns as that thing doesn't last long.Finally I would suspect the crank position sensor.It's in the optispark.Also check all the records you have for water pump replacement and distributor work that may have been done.When a car would just shut off while driving in the old days and after waiting for an hour or so,it was the condenser in the old distributor that was the culprit.I could be wrong,as i have been wrong 5 times this year.Do the cheap stuff first,and find an old school mechanic(corvette),as the new mechanics don't have a clue on how these cars work.Sorry for the rant.My02.

Injuneer

Before you start randomly replacing parts.... TEST FIRST. Check the fuel pressure. That will tell you if there is a problem with the fuel pump or fuel filter. Have you checked for spark when it won't start, or stalls, by pulling a plug wire and holding it near the block? Have the ICM tested at a shop, making sure they heat it up by testing it repeatedly.

How did the mechanic check for codes? The 94 is difficult to scan. Requires an OBD-1 scanner like the old Auto Xray, or (free) scanning software specifically for the LT1. Several recent dual (OBD-1/OBD-2) scanners will not work (e.g. Innova). And you can't short the pins on the ALDL connector and flash the codes on the SES light. 93 is the last year that would work. The fatal code for the cam position sensor in the Optispark distributor is DTC 16, and that does not turn on the SES light.

In addition to the ICM being sensitive to heat soak, the problem can also affect the cam position sensor in the Opti and the fuel pump. shbox.com has testing methods for many of the common problems, including the coil.

4th Gen LT1 F-Body Tech Aids

jbert55

Thanks for the great ideas, I will get to work on it in a day or so! I'll report back on the results!

Injuneer

Do you need the specs for testing the fuel pressure?

jbert55

I found "Checking fuel pressure loss" on the shbox.com site, is that correct?

shoebox

Sure is.

Fatdog2

Right again Injuneer.When you go to the doctor,the first thing they do is take your blood pressure even before you see the doctor.Same holds true with these cars.Fuel pressure check first.Why I don't remember this is beyond me.My bad.

jbert55

So I have been working on checking the fuel pressure..what a pain to get to the schrader valve!! Anyway, 32 psi at idle, 41-43 with the vacuum line unhooked, approx the same at WOT. Hard to be sure with a gauge taped to the windshield on our curvy, hilly roads!!
But, when I shut the car off, fuel pressure goes down quickly, under 20 psi in 45 seconds, 0 psi in under 2 minutes. Read thru the test on shbox.com, and not sure if I want to clamp the fuel line to test whether it is the regulator or fuel pump. Also wonder if this could cause the car to just quit like it does....

I also put a new coil on, as it is easy and (relatively) cheap. Drove the car back to town to return the pressure gauge, and it did it again..just quit. As I was coasting, I tried to start it, it fired and died, then just cranked. After I pulled over and stopped, I waited 30 sec or so, tried starting and it fired and died again, then after a minute, it started and ran fine. Freaky.

I will say that I filled the car up before I drove back home, it was down to about 1/3 tank (put in 11 gal) but not sure if this is relevant, just full disclosure here.

Injuneer

Fuel pressure sounds normal when engine is operating.

Rapid loss of pressure on shutdown generally does not affect the way the engine runs, unless it's the fuel pressure regulator, but it will extend the cranking time. The loss can be due to:

-leaking check valve in the pump
-leaking diaphragm in the fuel pressure regulator - check for wet fuel in vacuum line
-weak spring not shutting off return line fully on shutdown (agree, don't squeeze the return line).
-leaking injectors
-leaking fuel lines, including the flex line in the tank.

I'm surprised there are no codes. The Opti cam sensor and the ICM should set codes, if that's what it is.

There is also a possible issue with the ignition switch. Does the dash go dead when the engine suddenly shuts down? The contacts in the switch burn and make intermittent contact.

jbert55

No fuel in vacuum line, no visible leaks externally at all. Sits in clean, epoxy coated (floor) garage so I would expect to see any drips.

Dash does not go dead, idiot lights come on, etc. I will try to get it back to the shop tomorrow and check for codes again. Will see if he has an old OBD-1 scanner, we were using one of his newer, multi function ones.

Thanks for the directional help!

jbert55

Wow, busy month! (BTW, it is durn near impossible to find a mechanic with an old OBD1 tester...)
Finally got back to the Z, started checking the distributor/optispark. The unit on the car is a MSD, just installed last year, I don't think it is the problem. Spoke to the previous owner, and he said he also replaced the fuel pump last year.
He suggested that I check the oil pressure sensor for the computer, as he had heard that it could cause this type of intermittent problem. Sound reasonable??

Pullngz

Have you verified the function of the opti? You mentioned you started checking, did you completely check it for proper operation?

Does the car have an additional ignition box ie:MSD, Accel?