Chris Lockhart

Hey folks, about a week ago, this problem slowly cropped up, and then suddenly got worse. The car is basically undriveable now, which isn't good, as the GT is my daily driver.

The car surges badly, starting around 2000-2500 rpm with no change in throttle input. I thought it might have been a fuel delivery issue, but upon reinstalling my fuel pressure gauge today, it's reading around 60 psi at idle, and doesn't drop off with application of throttle.

I have already installed new spark plugs, cleaned the contacts and checked for cracks and/or evidence of flashover on both distributor caps and rotors. I've inspected the plug wires and have found no obvious defects. I also replaced the fuel filter.

It is not switching into 4 cylinder mode, as I have installed the replacement relay that eliminates the ignition monitoring relay. I did this because several months ago, it kept going into 4 cylinder mode, when there was seemingly no reason for it. I suspected that the temp sensor was bad, and due to cost, replaced the relay vs the temp sensor. (maybe something was slowly creeping up at that time???????)

Any and all help is appreciated, as I am getting frustrated and losing focus.

the flyin' scotsman

How's the brain/eproms?

shaaark89

mass air would be my guess.

blau928

Check the MAF...

atari_st

My 86 did the same thing. Started back firing...and loosing power for a couple of weeks. Finally I could not even keep the thing running. Soaked two sets of plugs in the process...spent two weeks trying everything I could to get it fixed. I tested the MAF per the SM and it tested fine. It got so bad...I could not even get the car to run at idle. Disconnected the MAF and it started. Connected it back...car would not run. Replaced the MAF and everything was back to normal.

cfc928gt

Agree with the above, try unplugging the MAF and see if it runs better, not good, but better.

Chris Lockhart

Sorry guys, I forgot to include some info in my original post. I did disconnect the MAF to see if it helped and it did not. With the MAF disconnected, the car would not idle at all. I had to keep the revs above 2000 to keep it from stalling. Reconnected the MAF, and it idles fine. Any other ideas, or could the MAF still be "marginal"??? I do have a local lister with another '89 GT that we could swap MAF's for testing purposes I'm sure.

Also, I checked the plug wires in the dark last night, and there was no light show. All looked normal. I listened to the injectors pulsing with a stethescope last night also, and although all are firing, none of them really sounded the same. Some were very strong sounding, while others were quieter.

Drmark

Chris, sounds like the ECU to me. I had a similar fault on my 87 S4 which did not respond to usual measures (including rebuilt MAF - it had done 90 K) so swapped brain in footwell with another S4 and problem solved.
Surging made it undriveable and it cut out completely if I backed off the juice - tricky in an auto but good for practicising left foot braking! Have you got someone locally you can swap the ECU with prior to sending off for a rebuild?
Cheers
Mark

John Speake

Doing the MAf disconnect without an improvement isn't an absolute MAF check. It could still be the MAF.
The MAf tests in the W/S manual are only good for gross failure.

But it might be partial LH ECU failure. Does the car run OK for the first couple of minutes ?

Regards

Chris Lockhart

John, the problem is present right at start-up.

Mark, I do have a local lister w/ an '89 GT. I will see if we can get together this weekend for some swap testing.

John Speake

OK, if the problem is right from cold start, then it is not O2 loop/sensor.

How many miles ?

Any 87--> LH ECU will prove whether it is LH or not. If you blip throttle with car stationary, and lets revs drop down, is the tach reading smooth, or jerky ?

Regards

MarkRobinson

60psi is WAY too high. S4's have a 3.0 bar regulator, so at idle, you should see 37-38psi with vacuum, & 45psi without. Because of all this extra fuel, the surging you're feeling is your O2 frantically trying to pull out injector pulse to compensate for the extra fuel pressure, so you're feeling the extreme rich/lean conditions on the motor, which makes it surge, even while driving. I've had this on 3 of my '85-86 cars since AA's chips allow too much injector pulsewidth at closed-loop, low rpm driving. Most of this surging is felt when holding the throttle steady at 1500-2200rpm (on my car). Because of your extreme fuel pressure, I imagine this is what's happening.

Chris Lockhart

John, it's got 137,000 on it. The tach reads steady. No jerking. I do have a sticky idle air control valve that occasionally lets it's presence be known with a high idle when the ambient temps are 90+F.

Mark, I think that you're off on the S4 fuel pressure. The lower pressures that you mention may be correct for an earlier model, but the S4 FP should be around 56 psi.( I believe the manual states 3.7 or 3.8 BAR) I upped it to 60 with the RRAFPR, because several folks on the list had determined that there was a decent power increase at that level in conjunction with exhaust mods. This was determined via dyno testing at the time, approximately 4 years ago, and I've had it set there ever since with no ill effects. Dyno pulls prior to my Hall sensor giving up the ghost were 303 hp and 300 tq at the wheels with this pressure.

John, that brings up another issue. Wally had put the Hammer on my car at SITM last year, and it showed a Hall sensor issue. I unplugged the sensor, cleaned the contacts and hooked it back up, but have not checked it with the Hammer or dyno to verify it's actually operational. Not sure if that has anything to do with my problem, but I figure the more data the better.

Thanks guys for hanging with me on this. It's much appreciated.

John Speake

If the MAF has never been changed at that milage, it's MAF is due for replacement !

I suppose a Hall sensor, if intermittant, might cause the ignition advance to kick around, but I don't think this is the problem here. You describe it has having a gradual onset.

See if you can borrow a good MAF.

MarkRobinson

Ah, I was just going via memory from seeing those numbers in the manual.