'88 will not idle and will not rev
 

First post in many, many years, but hoping for some troubleshooting steps for a no idle, no rev condition on my '88 S4 manual. Car sat in the garage for over 2 years with a no start condition. Finally got around to trying to get her back on the road.

Work so far:
Drain and clean tank, flush lines
Dead pump, so new fuel pump and filter
New in tank strainer and sending unit
Rebuilt injectors and replace cracked vacuum elbows, T's and Y's

Car starts now but requires throttle to stay running. Likes 1,400 RPM best, smooth with no misses, but give it more throttle and it gets rough and starts missing badly. Let it warm up fully with foot on gas and tried to slowly drop it to idle but it again starts running poorly and dies at about 1,000 RPM.

Have a new set of plugs to go in but doubtful that helps. Will take a bore scope to check vacuum connections under manifold and check fuel pressure regulators and mitivac test vacuum. Where else should I go next?

Trying to get the thing running for son's wedding this Saturday. Hoping not MAF as rebuilt 6 years ago or something else likely to take too much time to fix in time.

TIA

Swap in another MAF....

Isn't there some diagnosis to be made by simply unplugging to see if it idles or remains same?

Sure..

Based on your report I would remove the MAF and the LH and EZK computers and send them to be checked and rebuilt if necessary
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You could put your parts into a known good running car,
but I will bet this refresh work has not been done to your car.

So you can test the parts now or you can send them out later.

Note this will be the final result after you test them and find they are not working

Note this service needs to be done on all S4 series 928s

When my LH ECU failed it would run in a similar fashion, but only intermittently

So beware if it seems to fix itself, you could end up stuck somewhere

I would follow Mrmerlin's advice

Thanks for the input fellas. Been on the phone with Rog after disconnecting the MAF. There is no significant difference in behavior with it disconnected. He says to check the fuel pressure regulators next, then swap in a core MAF I have in my spares. Bore scope under the intake shows no other issues with rotted vacuum connectors.

Pulled Mitivac vacuum on both regulators and they are both rock solid, no leak down. On to the spare MAF.

Spar MAF actually runs worse, if possible. Anyone have a spare LH brain lying around? Don't have time to send mine for repair but really looking like this is my problem.

Old MAF's tend to under read air so no surprises if it runs a bit rougher with your spare unit compared to one that has been refurbed- but that tends to suggest the LH is suspect unless you have some dodgy wiring on the harness inlet to the MAF which is another common degradation issue on these things. Needless to say the engine management harness and the front of engine harness by now are typically falling apart or badly degraded and along with the LH and MAF units, all need to be in tip top condition to achieve a good running motor together with clean injectors.

Thanks Fred. Everything else that could result in this severe no run condition have been replaced or checked to the best of my ability. The fact that the pump will only run with a jumper seems to further indicate no signal is coming from LH brain.

If you removed the battery ground and reset the ECUs, yes, it will run rough.

If you didnt, the ECUs have fuel trim based on the old MAF and it will run rough, you should pull the battery ground to set a new trim baseline..

Roger at 928rus is sending me a loaner LH brain, a new MAF and new injectors. If that doesn't get me running, it will have to wait for another day. Can't thank Rog enough for his support.

Glad you were able to get some loaners from Roger.

I saw your email to the Houston 928 Group.

Hopefully all goes well after installing the spares.

Thanks buddy. Rog and I apparently miscommunicated on the LH availability, but as always, he is awesome and came through for me.

Best,
William