loopy

This 1994 starts good initially when cold, idles fine, but hesitates and sputters on acceleration. It has to warm up for a few minutes to overcome this. When the engine is warm, it is harder to start, it has to crank over a few times. It runs great otherwise.
It has an engine code 26 EVAP purge solenoid circuit, changed that out as well as the gas cap to no avail. Not sure I am getting a ground from the PCM when I should.
New distributor, plugs (ACDelco 41-906) and wiring harness installed recently.
I have seen similar problems on this site, but have not heard of any resolution.

Any suggestions would be appreciated.

Injuneer

Welcome to CamaroZ28.com!!

"New Member Welcome" isn't the best place for tech questions. Let's move this to "LT1 Based Engine Tech" for increased chances of responses.

Dtimekw

Welcome to the forum!

First thing I would do would be to create a "signature" outlining what your car has for example, auto/6spd trans, any mods it has like headers or deleted AIR. This will help us narrow down possibilities and draw from our experiences.

Hesitates and sputters? Is is backfiring or bogging down? Why was the distributor (optispark) replaced and what brand did you replace it with? By wire harness, do you mean the spark plug wires? Have you checked the basics like vacuum hoses cracked and leaking, fuel filter is always a good one for your symptoms, water in the fuel etc....

Did it happen all of a sudden or did it happen after changing or doing something to the car?

OK, lets start there. Again, welcome to the forum, you'll find a lot a great people on here with a lot of experience!

loopy

I bought the car late last year and ran it for one month before storing it for the winter (6 months). When I tried to start it in the spring, it barely turned over, almost as if it had fuel or water in the cylinder. Charging the battery made no difference. Pulled the spark plugs to check but found no moisture on plugs or in cylinders. Put new spark plugs in (Champion) and found out that we had this hesitation problem when accellerating and it would sometimes stall when cold. We changed the new Champion plugs with ACDelco plugs and new plug wires and new new distributor bought on Ebay (6505-PAC619R).
We replaced the distributor because we had previous moisture problems with a 1992 LT1 Corvette.
This is a stock 1994 Z28, automatic 5.7 L engine, supposedly no modifications.
The original spark plugs taken out were NGK-TR55GP.
We checked the vacuum pressure and vacuum hoses they seem OK, pressure is good.
Have not checked or changed the fuel filter because it seems to run OK when warm.
The car does not stall anymore with the new disributor, it hesitates in neutral when accelerating or when driving (bogs down) before it warms up. Have heard the occasional backfire when revving up in neutral, not normally there when accelerating. It feels like I am not getting enough fuel at that time.

Dtimekw

How did you check vacuum and what was the reading? Sounds like bad gas could be part of the issue. How much gas was stored and did you add good gas. I know things like fuel stabilizers and fuel storage additives are there but fresh fuel is the only way to go, at least you would be able to rule out bad fuel as the problem. If you can siphon some, put it in a glass jar and let it sit for an hour. This will check for water in the fuel, another problem from storing. Who did the opti install? It has been noted a lot that they can be put on wrong and cause issues. Shoebox just recently talked about it to a forum member who was sure it was put on correctly until he took it off and found out that it was not installed right. Can you clear the code and if so, does it come right back? I would also double check all spark plug wires are on tight and in the right place i.e. correct firing order. Don't rule out the fuel filter either. Its easy to change and again will rule it out. Any weird noises in the valve train or anywhere else for that matter?

We'll get there, don't worry

loopy

I checked the vacuum at the solenoid valve hose from the throttle body side, it is supposed to be at least 7" of Hg, I was reading 14" Hg on my old vacuum gauge.
When I left the car sitting for about 6 months, the tank was full and I had added fuel additive prior to this time. The car was running good before I stored it, but sounded horrible when I first tried to start it in the spring.
It did not even want to turn over normally and sounded terrible like there was fluid in the cylinders opposing compression (Hydorlyzing?) That is why we first checked and changed the plugs and after that when it became apparent that it was hesitating on acceleration, we changed the opti and wires + plugs again.
Drove it for a months in the fall, several fill ups before I has this problem, and since the spring I have filled the tank many times. I will check the fuel this weekend, I have purchased a fuel filter, which I will install after I check the fuel.
The Opti was installed by myself and my son, we are very careful in this type of work because it is a horrible and time consuming job. Everything was labeled, checked and rechecked.
I cannot reset or clear the DTC26 code with my OBD1 tool, it goes through the procedure but is not successful.
Not getting any weird noises from the engine, it sounds great.

Thanks for the updates, will get back to you.
bob

Injuneer

Do you have 12V to the power side of the purge solenoid with key on?

Try a full vacuum port like the EGR port on the passenger side. Should have 20" Hg at idle.

loopy

I do have 12 volt on the power side of the solenoid purge valve, with the key on.

To check the vacuum on the EGR side: it is not that easy to disconnect the very small non-flexible vacuum line on the 'driver's side of the engine. It looks like it should just pull off, however, I don't want to damage it.

Still have not replaced the fuel filter yet, waiting to get my car on the hoist at home.

A mechanic suggested from the symptoms described (hesitating on acceleration when cold), that it points to the EGR valve. Any ideas on that one?

bob

Injuneer

If you verified 12V at the EVAP solenoid with the key on, and the solenoid is new and "good" (check for 15-30 ohms resistance across the solenoid), then you don't have a good ground circuit from the PCM.

Shoebox has the wiring diagram. 94 is same as 95:

http://shbox.com/1/1995_pcm2.jpg

There was no purpose in changing out the fuel cap, since OBD-1 does not monitor tank pressure.

The EGR vacuum solenoid vacuum line has a rubber elbow where it attaches to the nipple on the intake manifold. That should pull off, and if it is damaged, you can get a replacement elbow at any auto parts store.

The EGR valve would be a problem if it was stuck open. It could make the car hard to start, and idle and run rough when it started. And it would continue to make it idle rough (or stall) even when the engine was fully warmed up.

With the engine idling, reach back and press the EGR valve open. The engine should stumble badly.

loopy

Hi Fred,
The EVAP solenoid looks good, I am probably not seeing the ground from the PCM, either wiring or PCM is not setting the ground. All the connectors are at the bottom of the PCM and I'll have to pull it to check for continuity.

I pressed the EGR valve open when idling (did not know you could do that) and the engine does stumble badly, I guess the EGR was not stuck open. The engine idles fine when warmed after a couple of minutes; it runs like a rocket when accelerating.

Bob

Injuneer

Did you try clearing the code simply by pulling the "PCM BAT" fuse in the panel at the end of the dash? Just need to leave it out for 30 seconds.

Running a data log with Scan9495 may turn up the problem with the hesitation.

https://www.camaroz28.com/forums/com...95-lt1-874306/

loopy

Pulling the PCM battery fuse made no difference, code appears again after starting.

Injuneer

Then you have to verify the continuity of the solenoid (looking at it will not tell you anything), and verify the continuity of the ground wire from the PCM.

loopy

The hesitation problem on acceleration when cold and the hard start when the engine has warmed up has been resolved. I replaced the Coolant Temp Sensor, mounted on bottom of water pump.
I still have the EVAP minor problem to resolve, still have not checked for continuity to the ECM. The car has been off the road for the last month.
In the meantime I have replaced the driver side Power Window Motor, which was 'tired' and kept stopping. Thanks to this site, it was an easy fix with all the photos and detailed instructions.

Eric94GT

Bad gas?

Edit: Never mind, saw that you got past the poor running.