I think I know what this is, but would like some extra help to be sure.

Engine starts and runs fine, all gauges in normal range. When cold, has occasional very short cutout, but power is normal otherwise. Runs good and has good acceleration. After fully warm, engine power is way down, have to hold more than half throttle to go 40MPH in 4th gear. Won't pull much of a hill. No smoke, no fumes, free rolls fine, clutch pulls fine, brakes grab and release fine. Once in low power mode, stays that way until shut off and restarted, then goes through same symtoms. When in low power mode, idle is below 600.

I have checked:

Timing is okay, right on 10deg at idle, advances to beyond 30 when revved

Fuel delivery is okay, listened to all injectors with stethescope, hear them fire.

Checked spark at all wires both sides okay.

Belt looks okay from vent holes.

Voltage at jump lug >14.6 VDC.

Fans cycle normally, temp stays around 185F.

Please let me know what to check or what you think it might be. The short cutting out has been going on for more than a week, and now has gotten much worse when warm. It just has no power to move, and sounds like it's struggling to stay running.

Thanks

Doc, 90GT

 

MAF?

 

Sounds like a fuel delivery problem. The fuel pressure when the engine is running without any load is substantially less than when under power. When the engine is cold the computer puts in extra fuel sort of like the choke in the old carburetor days but once the engine warms up the computer is trying to control the air fuel mixture based on feedback from the oxygen sensor. If the sensor is bad (or the wiring to the sensor) you could be running very rich or very lean.

 

Doc? You have access to a Hammer? Or another LH? (Has your LH been rebuilt?)

What measurements have you made - besides idle - when the car is in 'low power' mode?

 

If you have a "cold" piston you'll go into limp home mode. Check that all your ingectors/plugs are firing. If you pull one of the two center plug wires off on either side while the car is running and nothing changes you'll know that you're "limping" Also check the lights on the LH module to see whether they're showing a problem.

 

I can borrow a Hammer, and my LH hasn't been rebuilt. Haven't made any other tests yet, just happened tonight. Ernie, I'll check the center plugs using a inductive timing light. I checked the plugs for #1 and #5, and they were fine, I though at first I had a coil problem, but I have fire to both distributors.

I need to look up the lamps on the LH, I haven't done that before.

Thanks,
Doc

 

Check the injectors as well as the plugs. If there's too great a delta between the temp sensors on either side of the engine the LH will turn off one set of injectors, the two "outside" cyllinders on one side and the two "inside" cyllinders on the other.

 

Are you sure it isn't going into 4 cylinder mode from the ignition monitoring system? The symptoms aren't exactly right - power sounds too far off - but, pull the carpet and expose the ignition monitoring system and see if one of the LEDs lights up when it goes into low power mode. It does reset on shutdown, so you won't find the problem in garage testing. You may have a faulty thermocouple.

 

It was cold outside, so I might have checked the rest of the plugs with the inductive after shutting off the engine and starting again. They must turn off the fuel to the cylinders in 4 cylinder or limp home mode. It would be unsafe to squirt fuel if the spark were shut off.

I'll have to locate those temp sensors, and I'll check the LH tomorrow to see if the lamp comes on in limp mode.

Thanks guys, I'm hoping it's not MAF, that was my first guess, but if it stays in low pwer mode, I guess that's maybe not it.

 

Yes, the ignition monitoring system kills fuel to half of the cylinders when it detects a difference in exhaust temperature between cylinders served by each of the two coils, based on the idea that half the ignition has failed (bad coil, bad coil wire, etc.), although fuel issues could cause the same temperature difference issue. Anyway, the system's intent is to prevent unburned fuel from heading down to the hot cat converters. In your MY the sensors are under cylinders 4 and 8. You can see them from undeneath the car.

Just remove the carpet over the CE panel and drive around and see if the LED lights.

Red LED: Ignition Circuit I (cyl 1,4,6,7) - passenger side coil
Green LED: Ignition Circuit II (cyl 2,3,5,8) - driver side coil

If you verify that you still have spark while the system is tripped, you could bypass the relay by pulling it and jumping the plug sockets for the relay terminals 87, A1 and A2.

Mine tripped a couple of times, and I felt the power loss, but it seemed yours might be greater. However, my guess is this is your problem.

You can try loosening and reseating the sensors. They can be tested. They are over $200 each to replace.

 

Generally, when 1/2 the cylinders stop firing and the bypass relay LED lights up for limp home mode, you friggen KNOW it. It is not subtle. Sounds and feels like the motor is going to exit the engine bay.

 

I was heading up over Donner Pass in Lake Tahoe when the car downshifted to keep going uphill, but the car still moved along fairly well. I Knew something was very wrong. I had the carpet off the floorboards as I had just finished an open road race that day. I looked over and there was the glowing red LED. Pulled over. Stopped the car. Restarted and the problem did not return for a month. Initially I thought doc's sounded more severe, but, Dave, I believe now, this is a good potential explanation and needs to be ruled out.

 

Bill, you may be right. I've never had the stones to do anything with Limp Home Mode other than turn the lump off and fix it. I've not (yet) experienced it except at idle.

Doc's issue sounded - to me - less severe/obvious that LHM. I'd put my money on LH or Hall Sensor. The 'cutout' phrase Doc used screams 'electrical'. Maybe final ignition stages too. They could get temperamental when warm. But, I'd expect them to be digital. Work or not. Since Doc's got a 19-pin connector the lazy approach is to steal a Hammer from Roger for a few minutes.

 

Doc,
Do you want me to come over with the Hammer?
Roger

 

Carefully check the coil wires, especially the one on the driver's side.

It is also possible (but not too likely) that the problem is a partially plugged exhaust.