this one's driving me nuts, so i though i would ask here where there is more action on the 964 forum.
1990 C4 cab, starts fine cold, but within a few minutes, it gets into a rough idle if left to idle, or if driving, totally looses throttle response, but it will still idle if you push in the clutch as you're rolling along. if you turn the key off and on again, the engine regains power and you can drive normally for a few seconds before falling off again. this is infinitely repeatable, so i always make it back to the shop.
first thing was to replace the DME relay and head temp sensor. no love.
airflow meter was swapped with a known good one. swapped a known good computer next. nada.
fuel pressure check with a long hose so i could watch the gauge w/driving, along with the hammer on the seat. pressure never varied. compared hammer readings to another 90 that was here. pretty much the same readings.
the only DME fault that stays the same is "HO2S short to B+". not sure what the short to B+ issue is, but did it with the other computer too. running with the O2 sensor unplugged didn't change the running issue. new O2 sensor.
installed a flywheel sensor for the hell of it. nada.
fuel filter clear. loosened gas cap for possible tank vent issue.
voltage normal. battery terminals checked and clean.
sent out the injectors to be cleaned. nada.
no vacuum leaks.
removed exhaust system. found the aft end of the cat was starting to unravel, probably from this running issue. popped on a used one to test. nada. ran cat only, no muffler. same.

kind of running out of ideas, so has anyone had a problem like this? any input would be appreciated.

 

Have you checked the distributor belt ?,when that fails you get power loss.

 

yes, i forgot to mention that. the caps and rotors are good and both rotors point at the proper timing tit on the distributor body. belt failure doesn't cause the 95% power loss i'm getting though.
i also borrowed a pair of ignition coils and amplifiers, but still no difference.
the clue, i'm pretty sure, is what causes it to run well for a few seconds after the key is turned off and on. you can do this while coasting after each power loss episode. same at idle. rough idle, shut off and restart, and good idle and a few seconds of accelerator response.

 

Did you see the same O2 fault with the lamda probe disconected ?
Did you look at actual values and check lamda operation?

Looks like the Hammer could be telling you whats wrong !

Geoff

 

yes, same fault, along with HO2S not present when disconnected. same with the loaner computer too. kind of unusual to have a short inside a wire loom that doesn't flex. generally the problem is at the ends.

 

W,A, Guess , bad ground .

 

i know it's going to be something stupid, whenever i find it. i've checked every ground in can find, guided by the wiring diagram, but it's a possibility. it was suggested by tyson on the pelican forum that electro-magnetic interference from old plug wires could cause this. i haven't experienced an almost total power loss like this from plug wires on anything, yet. wires still look good, not oiled, soft and pliant, but could be. the flywheel sensor is new. the halls sensor is original. the short to B+ fault remains with the O2 sensor unplugged and with the loaner computer. makes you think it's in the loom, but wire shorts inside a non flexing loom are very unusual. they're generally at the ends of the loom.

 

This problem is not too uncommon for a 964. It's been answered a number of times on Rennlist,
where the real problem solutions are found and not the typical rap of the "other" forum.

Do a search, and start here:

https://rennlist.com/forums/964-forum/237038-running-lean-faults-internal-control-unit-and-cylinder-hall-signal.html

And then check here, www.systemsc.com/diagnostic.htm , under Poor Running.

 

loren, any ideas, other than the links?

 

Backstepping!

How/when did this problem surface?
Was anything fixed/changed before this problem appeared?

Bill
90C2

 

first noticed when driving to seattle from california. just a couple of short episodes then. it's taken about a month and several tanks of gas to get this bad. no recent previous work done down there.

 

this is a long shot but solved somewhat similar problems I had recently: Remove the negative battery cable and clean the cable-to-car connection thoroughly. Car ran like a top afterwards. Before it was idling in a really weird, erratic way, bucking, snorting with intermittent running normally, sometimes coming close to dying.

 

thanks, but that's been done.

 

The "HO2S short to B+" I'm guessing, this means the O2 sensor lead is shorted to B+ or positive 12 volts. This condition would be there even if the oxygen sensor was connected or not. With the sensor disconnected what is the voltage at the connector?

Bill
90C2

 

Can you disconect the o2 sensor wires at the computer ?