wescraft1

OK , after much, much reading, I have broken down and given up on the exact answer, so I'm hoping you guys out there can help me out. I had my LH computer re-built and installed but now I have a very low idle (600 rpm?) and the car has been getting horrible MPG, along with a big banging misfire at cold start. It runs really rough until it's warm, but even after warming up the misfires continue and shake the car badly unless I downshift into lower gears to get the revs higher and keep the fuel pressure up. Is it the MAF? And if so, can I clean it at the port? I love this car. I don't have a lot of $$$ right now. Please... anyone who wants a loyal friend indebted for massive quantities of beer... Please help!

wescraft1

PS If it's something so simple as detaching the battery and then re-attaching the cables I might cry.

dr bob

Find another owner locally and verify that the problem isn't with the 'rebuilt' LH controller. Try your brain in their car. The symptoms you describe are consistent with LH failure.

Where did you have the brain rebuilt?

dr bob

Oh-- Try the battery cables disconnected for half an hour. Nothing to lose!

Jadz928

MAF. Bum a MAF to swap.

928 at last

If the half hour disconnect does not help, pull the negative cable off the battery again, and then VERY CAREFULLY check ALL your connections/connectors. Make very sure they're clean, in the proper place and tight. Pay particular attention to the connectors for the MAF and Brains. I'm assuming here you're happy with the condition of the grounds.

That in and of itself should take a half hour at the very least. Once you're happy with the connections, hook up the battery, and then try again. It will take the brains a few to "learn" once power is reconnected so don't give up right when the car starts.

If that doesn't help then start looking for a test bed for the LH.

Mrmerlin

pull the vacuum lines off of the both dampers and the fuel pressure regulator smell them for gas,
if you smell gas then the part is leaking you can also attach a mytivac to each part and see if they hold vacuum, the dampers are on the left front and left rear fuel rail the FPR is on the RR fuel rail to test the rear parts the aircleaner needs to be removed.
Also pull the coil wires off of each coil and inspect both ends and the the wires for chafing or corrosion .
report back what you find

TheoJ

hi Wescraft1, what MY do you have? It helps making a signiture.

Running rough might and misfiring may be something in the ignition, not even related to the LH repair. First try to make sure it is firing on all cylinders. Once a coil-distributor cable came loose and caused similar problems. If you have a IMR (Ignition Monitoring Relay, MY 89->) check that one and see if red or green LED is on. Maybe it switched off injection on one batch of injectors.
If ignition is perfectly ok, you focus on the repaired LH. A bad LH will make the car run not at all, or roughly when warm, low on power, bad fuel economy, sudden salls etc. That does not sound like your problems. 600rpm is a bit low, but not that much.

MAF problems will make the car sluggish, hard to start, run very rich or very lean but run more or less stable when you hold the gas pedal in. No misfires.
DON'T try to clean the MAF unless you know what you're doing. I did a serious number of MAF repairs. The MAF is a sensitive piece of equipment and damaging the hotwire makes it almost beyond repair. If you suspect the MAF and think it is not working but the car still starts, try this: remove the airfilter cartridge so that you can look into the MAF (grid). Switch on ignition. If you see nothing, you're probably ok. If you see the MAF wire glow red hot, you have a MAF repair on your hands. Looks like this:


Battery disconnect is perfect, easy to do, but as the LH was repaired I presume it is now peoperly reset.
Who did the LH repair? Maybe you can ask for a verification that yours is really perfectly ok before persuing other options in depth. Maybe someone can swap LH for a few moments?

regards

Mrmerlin

with a questionable LH you would want to put it in a known good running car . This will prevent putting a good LH into a known problem car and possibly damaging it

Rich9928p

Disconnecting the MAF and running the engine puts the LH in a limp home mode. The engine will run with the MAF disconnected. However, if the engine is cold, you may need to hold the throttle open when cranking to start the engine. This allows the car to be driven to a repair shop. The LH defaults to the follow pulse width:

RPM < 2000 RPM: 3.5 mS
RPM > 2000 RPM: 6.3 mS

With the limp home mode, you can imagine there are just a few RPM regions where the injection pulse provides the proper fuel mixture. However, unplugging the MAF provides a good test to see if your MAF is dead. If the engine runs better with the MAF disconnected, it is likely your MAF is not functioning.

wescraft1

Gentlemen,
Thanks so much for both the quality and quantity of your replies! I am headed to the shop with instructions in hand tomorrow.

Many thanks,
WRC

PS It's an '87 auto - and it's a beauty.... The car of my dreams since "Weird Science" came out in the theaters way back when.

Landseer

The LH is grounded at the back of the valley of the engine, difficult to get to, under the fuel regulator and cable. If the ground is corroded it can be the cause this type of problem. It actually shares a pair of ground points back there with other critical engine electronics. Part of rehabilitating these cars always includes cleaning that ground point.

Separately, totally different direction: has the bell housing been dropped yet on this car and the flex plate tension released and, importantly, the crankshaft endplay been measured? If not, that needs to happen and you will likely need an auxilliary clamp fitted to counteract a design flaw that has been documented on S4 automatics here in the later years.