Hi all, my 89 740 has a wierd problem. Sometimes when I turn it on, it runs like garbage in boost. Almost like the AMM is unplugged, but only in boost. You can baby the car up to speed, but as soon as you hit boost, it stumbles right away. When I did actually unplug the AMM the car ran like crap any time above idle. When this problem is going on, if I lift off the accelerator, the needle on the boost gauge drops back down to normal and the car runs and accelerates fine until it hits boost again. If I hold the pedal in that position the car will begin to slow and I get popping from the exhaust, but the needle on the boost gage stays where it's at in the boost area. It acts alot like there's a leak in one of the intake hoses, BUT if you then turn the car completely off and restart it, the situation will either improve drastically or completely dissappear and usually remain gone until some random time in the future when you go out and start your car again. The times that it just improves but doesn't go away completely, it seems like the car can't breathe right. It will boost up to a point, it doesn't run rough, it just won't go like it should. The problem never just goes away, you have to actually turn the car completely off, and then restart it. Because of this, I'm convinced that it's an electrical issue, but I went ahead and changed all of the rubber hoses from the AMM to the throttle body, inluding the short one that connects to the turbo, I've swapped the computer, and I've unplugged the O2 sensor. At first, I thought I had found the problem. The car ran great for a couple of days, but then this morning when I came to work, it did it again. So now I don't know whats going on. I've tried searching the forum, but can't find any other posts. It doesn't matter if the engine is warm or cold. Could it be the AMM? I bought a used one and tried swapping it just to find that the one I bought was DOA, the car wouldn't run above idle. What am I missing?

Your car is LH2.2 for the record. From what I'm hearing, I wouldn't think the issue HAS to be elctronics. You could have a broken up cat. Hold your hand near the tail pipe when it's running poorly and see if the flow seems obstructed. Have someone blip the throttle a couple of times (or try to) and see if particles fly out. Sure signs of bad cat. I've had chunks that would fly up under boost and drop back down at idle. You could have a leaking I/C allowing unmetered air into the engine. This would lean the mixture, the O2 would tell the ecu to richen it up...viscious cycle!. Not enough fuel could do the same thing; a bad fuel pressure regulator (pull the vac hose off and see if idle rises, it should if working. Also, smell the vacuum hose for fuel. Anything more than the vaguest of smell means the diaphram is bad). You could have a bad fuel injector/radio suppression relay under hood, a failing fuel pump, clogged filter(s). Are you running more than factory boost levels? How are the ground wires on your fuel rail? They get loose/corrode and cause havoc w/ injectors. Have you checked to make sure the TPS is set correctly? Under WOT, the ecu basically dumps all the fuel it can into the system...if the TPS is bad/set wrong, the ecu could possibly not get the command. Oh, your ecu could be dying but no big deal, those 541's are very common on eBay. Good luck! Let us know your progress!
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When Volvos are outlawed, only outlaws will have Volvos!!

If the Cat was bad, would it bog down when revving the motor in park? Would the problem go away when the key was turned off and back on? It only stumbles when there's a load on the engine, even then it idles fine. I will check the I/C, the FPR, and the relay. The outboard pump and filter are new in September, the intank pump and screen aren't new, but it makes no difference if the tank is full, half full, or near empty. I'm running factory boost. From what I've read here the TPS seems to be set right. It clicks right as the throttle opens. I have already swapped out the ecu with no luck. Thanks for the tips, I'll keep you posted as to what I find.

Maybe try disconnecting the knock sensor plug to see if things stabilize. "Popping from the exhaust" is usually a timing issue. Or it could be misfiring causing unburned gas to be pumped out the exhaust. ALL turbos require Bougicord plug wires. If you don't have them start with that.

Well, I unplugged the knock sensor and it still does the same thing. Start the car, runs like crap (just like limp home mode), shut car off and restart, runs better, shut it off again and restart, runs like there's no problem. It almost stranded my wife today in traffic, on a bridge, with no where to get off the road to shut it off. The problem is getting worse now. You'll be driving along and all of the sudden, when you try to accelerate, it bogs down. I'm about ready to get rid of the thing. When it runs right, it runs great, but I can't afford to keep changing parts that aren't defective. It's so frustrating.

I'm going to make two random suggestions. I don't know what exactly changes in the fuel injection strategy in boost but would guess there needs to be more fuel (to cope with the extra air) and/or a change to the ignition timing to prevent knock. So things like fuel pressure regulator (especially if it has a vacuum connection) and whatever LH2.4 (or is yours 2.2?) uses to adjust timing (knock sensor, etc) would be worth checking. Popping in the exhaust could be unburnt fuel combusting downstream of the exhaust valve.

Zack
1980 245DL M46 293k
1988 745T+ M46 238k

Well, I went to the Pick N Pull today and picked up a good AMM. In park or neutral, the engine revved very smoothly, and I noticed an improved idle. However, when I backed out of the driveway, and put it in gear, it bogged down the instant the needle hit boost on the gage. When I shut the engine off, and restarted it, it took off like a rocket. One second it was acting like the intake hose was split, the next it was fine. To summarize, all new intake hoses from the AMM to the throttle body, swapped ECU, unplugged O2 sensor, and swapped AMM. What else am I missing?

Bad CBV?
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Are the CBV's electric? Wouldn't this problem be continuous and not intermittent? It never shows up during operation of the car, only upon startup, and then usually goes away when the key is turned all the way to off and then back on.

My own car's engine was backfiring on downgrades in December when the engine was losing power on the highway while towing a heavy trailer over a distance of about 100 mi. I think that the radio suppression relay was gradually failing at that point and causing that. Maybe that was causing misfiring? My 2¢.

The two times my car was losing power in boost in earlier years, one time it was disintegrating foam on the inside of the air box being drawn into the intake manifold, I found out, and the other time it was something wrong with the wastegate - the car was being commercially maintained back then and I do not know much detail on the second cause.