3800 supercharged cruise issues (goes so lean it drops timing and cuts out) 87 GT (Page 1/1)
olduserswitchedemail JUL 13, 05:01 PM
Been a very long time since I have posted, couldn't get my original account back up because I had switched emails, so with that, background on my car

3800 l67, VS cam, 3.8 pulley. Nothing super outlandish.

ZZPerformance PCM

4 years ago started a 6 speed f40 swap and just got this done this spring (lotta stuff happened in life to drag out the project).

I used to use a AUTOTAP for a scan tool, but no longer have that. Have a very basic scan tool right now.

Anyhow.

Car had sat for a very long time which caused the gas to go bad. Really bad. Enough so that it gummed up the injectors.

To get it running I replaced all the injectors with original ones, and inline filter and the regulator., drained all the gas, started with fresh. Car then started and ran fine

Prior to the transmission swap car ran fine.

Now. Everything runs absolutely fine except when at just small throttle cruising at 70mph down the highway. Can watch air fuel realtime with PLX wideband monitor. As your cruising it will randomely go super lean, start stuttering the engine and timing will retard back to about 6 degrees.

If you give it gas or step on clutch or downshift it will come out of it, but again, at cruise it will randomly come back

I am lost as to where to start. I had another MAF on hand so tried that, no love there. Tried to find a vacuum leak, can't seem to find one anywhere. TPS seems to read fine.

Another weird thing it is doing. The fuel pump relay gets hot. All the connections are good, I noticed this, put a new relay in along with a new connector pigtail, still gets hot.

Because I had access to a post hoist so I could drop the tank I put an new Walbro 255 pump and filter in it, didn't make any difference with anything. Still gets the relay hot, still cuts out. Don't think the two are related, but am scratching my head on this one

Many thanks
Frenchrafe JUL 14, 04:43 PM
I've been through 4 MAFs in my time.
3 stopped working and/or fried!
Can you check that the MAF is actually giving a g/sec signal that varies with engine speed? And that it's not just dead and the PCM goes into limp mode?
Pretty much any cheepo OBD2 reader from Ebay or Amazon will do the thing.

------------------
"Turbo Slug" - '87 Fiero GT. 3800 turbo. - The fastest Fiero in France! @turboslugfiero
https://youtu.be/hUzOAeyWLfM

olduserswitchedemail JUL 14, 09:22 PM
Tried two MAF sensors, both work via the OBDII Link thingy I have to read. First one read ok, but I have had problems with MAF sensors in the past and had a second one on hand (literally had so many problems with MAF sensors I carry a spare in the car), neither helped.

Thank you very much though,
Frenchrafe JUL 15, 03:55 AM
Yeah, I had to do the same with keeping a second MAF as a spare. (The second one would work enough to get me home!)
For me, the real problem was voltage spikes of the alternator/regulator just making the MAF signal go too high for a few milliseconds and then the PCM would throw a code and go limp.
This unregulated voltage (17V spikes) would eventually kill the cheap chinese MAFs.
Without a really sensitive multi-meter, you could not see these voltage spikes? It was only when I learnt to datalog that I could see the these voltage spikes lined up with the bad MAF signals going too high.
(Interesting fact, the factory charge gauge needle would jiggle when the alternator started to spike. Since it's analog, it shows whats really there and not some average value.)

Long story short: after finally fitting a new alternator I have a better running engine and no more dead MAFs

If you are interested, I made a video about it:



------------------
"Turbo Slug" - '87 Fiero GT. 3800 turbo. - The fastest Fiero in France! @turboslugfiero
https://youtu.be/hUzOAeyWLfM

[This message has been edited by Frenchrafe (edited 07-15-2024).]

olduserswitchedemail JUL 15, 08:07 AM
Interesting, Never thought about that. It gives me a road to walk down. Sincere thanks. It will be a few days before I can play with it again, but I will keep this updated. Again, that alternator angle is one I hadn't considered at all. Didn't even think to think about something like that. What are you using to "datalog"?

[This message has been edited by olduserswitchedemail (edited 07-15-2024).]

Frenchrafe JUL 15, 10:13 AM
Yes, I would never have thought about a bad voltage supply causing so much trouble.
The datalogging comes with the OBD2 application that I use. (Can't remember which one for now; I'll post the name later with an edit.)

------------------
"Turbo Slug" - '87 Fiero GT. 3800 turbo. - The fastest Fiero in France! @turboslugfiero
https://youtu.be/hUzOAeyWLfM