So the story goes - I just replaced the Opti with a Delphi. Car had sat for a year. I siphoned the old gas out and put new 93 octane in. Car fired up and ran great, got warm, shut it off. Fired it back up and it was surging at idle and backfiring, trying to stall. No vacuum leaks. I suspected the spliced wires on one of my o2's were making contact with each other inside their oil soaked electrical tape. I cleaned all that and the car performed great for a few days.

Now randomly, sometimes I fire the car up and it's doing it again. Backfiring, loading up, surging. It seems to happen only when the car is cold. Once the temp gets up around 170-180 it all smooths out and goes away. If I let the car warm up in the driveway while it's running all nasty, shut the car off when it reaches temp, restart it and it's running great.


So for the record -

'94 Z28 M6
pacesetter longtubes/full exhaust and mailorder tune been on the car for 7 years and ran great.
Recent - delphi opti, meziere ewp, new coolant temp sensor, fuel filter



Am I right to think o2's might be bad? I've owned the car 5 years and it's only had about 4k miles put on it in that time. Car has done a lot of sitting around w/ random broken parts. I know there are no vacuum leaks, I don't suspect plugs/wires because once its warm it runs great, coil checks out with low resistance. I can only think of 3 things it might be - ICM, o2's, fuel pump.

 

Check the coolant temp sensor in the water pump. If it's not telling the ECM the engine is cold you will not get the required higher idle RPM and richer A/F ratio. Shoebox has a procedure for checking the wires and sensor.

 

I will check it via shoebox's website but it's a new sensor and the idle is raising when cold. It sits about 1500-1600 when cold and stays there about a minute then slowly comes down to about 800.

 

If it's related to a cold engine you have few choices..the temp sensor in the water pump, or it's wiring, is the primary culprit...as Injuneer stated before. Just because a sensor is new doesn't mean it's good.....where did you get it?

The ICM doesn't get better as it warms up...often the opposite. The O2 sensors are ignored while the engine is warming. The fuel pump is also not going to change as it warms up.

If you don't believe it to be the sensor, put a scanner on it and see what the PCM sees. you can download scanner software and do it with your laptop.

Post up what you find either way and we'll help....

Good luck.

 

Coolant Temp Sensor is new from AutoZone. I know... it very well might be bad. It does manipulate the gauge in a correct manner though.

I'd love the ability to scan it but unfortunately I've been out of the LT1 game for so long IDK how you guys are scanning them for codes. I have seen the SES on at times... other times its out. I'm sure there are codes in there!

If using laptop - where do I get this software? Which connector to I use?

Is there a handheld scanner like an Actron or ScanGauge I can use or find on the cheap? I don't need tuning ability just ability to pull codes. I can fix whatever is wrong mechanically once I get a code.

Thanks!

 

OK, firstly on that sensor, if the gauge is involved we're talking about two different things. The coolant temp gauge is connected to the sensor in the driver's side head. This is not connected to the pcm and it doesn't have anything to do with the problem. The sensor we're talking about is on the water pump housing..you can see it frpom underneath the car. That one is connected to the pcm and gives it the information used to set idle speed and mixture. Buy one from GM...Autozone is good for some things but I wouldn't buy sensors there.

If you're lucky you can find someone near you that has a scanner. I have a SNAP-ON MT-2500 scanner that tells you what's going on and will let you log it. It's old school but it works....it even works on my new stuff too.


If you can't find anyone around, you can download Datamaster from TTSpowersystems.com It'll let you monitor all sorts of parameters from a dashboard. You download it, get 20 free sessions and then, if you like it, you buy it. I think it works great...

For a cable you'll have to go to tunercat.com. There was another guy who built them but he now sends you to their page. Anyway, you can get a USB cable for about $85.

The issue may not be setting a code. That's why Injuneer suggested testing that sensor and I suggested scanning. Then, there are codes that set that don't illuminate the SES. It's all in good fun....they wanted to keep us on our toes......

Good luck!

 

I still haven't dug into this thing... maybe next week sometime. Any further ideas?

Gonna go ahead and order a new coolant temp sensor for the W/P housing from RockAuto. Anyone know which one is OEM? They list two Delphi sensors and one is listed as OE but the other has a heart next to it meaning it's bought most often. IDK which is to get.

DELPHI Part # TS10078
OE No 25037332

or

DELPHI Part # TS10075


any help?

 

The first one listed is for the gauge....wrong one.

The GM part number is 15326386 The AC Delco part number is 213-310? This info is courtesy of Shoebox on shbox.com

 

More follow up:

W/P Temp sensor is on order.




Saturday I drove the car around for an hour, it was fine. I haven't changed anything since the above issues, but I wanted to get all the old gas burned up so I was driving around my neighborhood.

I later came out and got back in it to go get fuel, made it to the gas station and filled it up. Kept driving around for a while and once the engine got up to about 190*F it stalled. Car wouldn't restart, just cranked. I ran manual fan switch for 10 minutes, tried it and it fired right up. I made it 1/2 way home and it stalled again. I waited again, and it eventually got me home after stalling out one more time.

I know this issue is already pretty well covered in the advice above, just wanted to add to the story for referrence reasons.

I'm not sure if the W/P temp sensor could cause the engine to stall when warm or if that would be more like an ICM/Coil issue. Anyone know?

I'm going to first swap in this OEM W/P Temp Sensor and see that that does.

Thanks!

 

Changed w/p temp sensor with GM OEM one.

No change.

Changed ICM with a GM OEM piece brand new.

No change.

Car just periodically runs great, other times runs terribly. I'm wondering if my oil pump or oil pressure sensor are failing? IDK!!!??? HELP!?

I disconnected the battery thinking the computer may need a reset since installing the new w/p temp sensor and ICM and wouldn't you know it as soon as I reconnected the battery and hooked up a fuel pressure gauge to watch what the fuel pump was doing and restarted the car, it sat and idled for 45 minutes without any issues. WTF!?

Last time the car stalled it was "dieseling" and almost sounded as if it was pushing air out of the intake... it was making this awkward hissing noise and dieseling then stalled. Since then I can't get it to duplicate.

Going to go home today and beat the **** out of the thing up and down the neighborhood streets until it stalls or gains some reliability.

So it's now gone from a cold start issue to an issue only when warm....

Oh and more details:

fuel pressure 42psi w/ key on
40psi while cranking
34psi at idle (raises some when you blip throttle)

oil pressure at cold start is 40 as indicated by dash gauge
once warm oil pressure around 18-20

battery charging 13.9v at idle.

 

Oil pressure sensor will not affect how the engine runs. It's only connected to the dash gauge.

If you have "warm oil pressure around 18-20" at idle RPM when warm, its OK. As the RPM increases, the oil pressure should also increase.

Did you check the coolant temp sensor, using Shoebox's guide? That would have told you whether it was working right, and prevented wasting money on a new one. Even if it's a cheap part, why spend money on parts you don't need? That's why people get outraged at dealers.... they spend your money playing parts replacement roulette. You always lose.


Resetting the PCM clears the long term fuel corrections (and not much else). These are learned from the O2 sensor feedback. If clearing the PCM caused it to run better, it may be pointing to problems with the O2 sensors. If the coolant temp sensor was faulty, it may have prevented the PCM from going into closed loop, which is where the PCM "learns".

Your fuel pressure looks OK. You also need to check it UNDER LOAD, at WOT, above 5,000RPM. Should not drop below 40psi.

 

For now... knocking on wood... problem has been located and solved!!!!


The engine just kept cranking when I went to drive it today. I got out, jiggled a bunch of different wiring sections and harnesses and the engine then fired up. It idled about 5 minutes and stalled. WTF!!!?? I was going crazy. Eventually the car restarted. I began wondering if the IAC valve was bad or something. I was messing with that connector and the car was fine, I bumped the opti harness wire loom and the car stalled. I got it restarted and went straight to the opti harness, as soon as I touched it, it stalled again.

It ended up being the Opti harness connector on the passenger side of the intake, where the 8" opti harness connects to the main wiring. It wasn't a bad/corroded harness, or broken wires, it was apprently just pins not making contact inside the harness. To correct this I bent the pins downward a bit with a small screwdriver and felt the connector TIGHTLY fit them and snap into place.

I restarted the car and noticed it didn't need to crank at all, it fired right up! Awesome, jiggled it all very rigorously and it never stalled. I unplugged it and it promptly stalled. Plugged it back in, jiggled it all a couple good times, went out and romped 30 miles on the car with never an issue.

 

Stepping back into this with a follow up and my hat in my hand...

Boy did I fail.

I'm not even certain I've fixed it STILL, but I have a smile on my face today and I haven't even wanted to look at the Z28 in months.

Upon the November entry above, I pulled the pins out of the connector of the opti harness and I hard wired them directly into the OEM side of the harness, eliminating the plaguing suspected loose harness. Car ran great until one day I was out doing x-mas shopping with about $600 worth of presents in the car and it left me stranded in the snow.

You lousy mother @#(!)#$!(*&()#!@%)*(!!!!!!!!!!!!

Then back around March I traced a bunch of wiring and thought I'd found internally broken wires in the main wiring loom. Corrected it, went for a test drive and 10 minutes into it the car left me stranded again, this time I was lucky enough to get a nice fella to tow me home since it was only about a mile on the other side of the neighborhood.

So that car has sat for the past 4.5 months. Taking up it's space in the driveway.

Allllllllllll this time. All this frustration and money and time!!!!!!




Ready below for the suspected fix.

 

I got home today and tried the car and it would crank but not fire. So I'm checking grounds out of boredom and because I'm tired of the car being a giant driveway ornament and notice a fat red ground wire from the ECU that runs like 4" to ground. I realize I've never cleaned that one so I remove the bolt and the ECU is now loose. So since I've been suspecting the main wiring loom, I flip it over to inspect the connections of the 4 harnesses into the back of it... one harness practically fell out. I inspect closer, the tangs that hold that harness in are broken and missing. Let me state - I'D NEVER SUSPECTED ANY ECU PROBLEMS BECAUSE OF THE PLETHORA OF INFO I READ HERE AND OTHER PLACES THAT STATES ESSENTIALLY "THE ECU WILL NEVER FAIL UNLESS THE CAR IS UNDER WATER" SO I NEVER EVEN TOUCHED THE ECU BECAUSE OF THAT STATEMENT. I got 2 zip-ties out and secured the harness into the ECU good and snug, cranked the engine and viola. Fire breathing dragon awakes.

I only idled/drove down the street and back, less than 1/4 mile so far, but I think and DEAR GOD I HOPE my problem is solved.

Now the sketchy detail - noticed on the underside of the ECU there was bright orange paint and a sticker had the two halves of the ECU sealed as if it was a temper seal. Sticker is old and faded but I made out a few words like "remanufactured" which scared me and I quickly shut the hood and hoped the bitch is now fixed. IDK what ECU is in my car but other than those two details, it appears stock in size/shape/location.

One other detail, the drip rail on the cowling area ends just atop the ECU and the rain water drips right down onto it!!!!!! WTF. I'm gonna get some metal flashing and divert it away, that's likely what caused those harness tangs to break away from the ECU. The harness is a weathertite connection so hopefully no water gets in there for now... I only noticed all this because yes, I was checking grounds in the pouring rain. I said I was bored... lol


Hopefully this is the last update/follow up on this thread and I sincerely hope it proves helpful to someone else in the present/future. Godspeed my american v8 brother, godspeed.

 

Zip ties seemed to not do the trick, car left me stranded about putting less than 200 miles on it.

I got it home and figured out how to safety wire a coat hanger around the ECU harness that has broken tabs. 35 miles on it and counting.

At this point I'm questioning the ECU. I dug further and found the sticker says PCMforless.com on it. I knew it was an aftermarket ECU but now I'm wondering if it's failing.

I may opt for a Reman. ECU from NAPA if the car leaves me stranded again. I'm tired of having no faith in this machine! If I can put several hundred miles on the car without incident I would be much happier.