Folks,

My '88 765T has developed a problem where it blows black smoke under certain conditions. It is easiest to get it to do this when I'm cruising on the freeway at moderate speeds -- say 60 mph. If I apply a slight amount of pressure to the accelerator, the car actually slows a bit, and the exhaust will emit black smoke. If I apply more pedal, it then snaps out of this, and behaves normally. No black smoke, either.

I have removed all the turbo hoses and manipulated them, looking for even the smallest of holes. They all appear to be fine. So do the various vacuum lines, as far as that goes. While I had the hoses off, I tried spinning the turbo's impeller. It spun smoothly and exhibited no play that I could detect.

I noticed, though, that the fuel pressure regulator appears to have a slight fuel leak -- it's hard to tell where the leak is coming from. This got me to thinking. I know that an fpr can fail such that the fuel pressure will be higher than normal. If this should occur, it seems to me that an over-pressure condition can result in too rich of a mixture. I don't have the equipment to test my car's fuel rail pressure, so I don't know at this point if the fpr is a problem or not. It seems, though, that it could at least be a candidate.

I'm curious as to what others here might think. Besides the above, is there anything else I should look for?

Thanks in advance,
Michael McBroom

Thanks, guys, for all the suggestions. I have printed them out and will print out the 700/900 faq as well.

I just replaced the old fpr. Unfortunately, it hasn't made a difference. I'm noticing now, though, that there is less black smoke when the a/c is off. I can only get it to blow black smoke with the a/c off if I push the pedal to the floor. Boost is good, as is acceleration when I do this.

I also just got off the phone with John at VPI, who tells me that an AMM will often cause the EFI to run rich when it is beginning to fail. I have a good one on my 745Ti, so I'll try swapping them before spending the $250 for a new one. If that doesn't help, then it's to the faq and your responses.

I'll let you know what the problem was after I fix it.

Best,
mwm

READ MY POST BELOW! Mechanics like replacing the AMM. If you have a bad connection, you can momentarily cure the problem when you disconnect and connect again the wire harness to the AMM whether you replace the AMM or leave it in place. Still, the problem will come back sooner or later. Check voltage and resistance for AMM (and other sensors, like ECT) against specifications and replace only as needed before you go throwing parts at a problem on the advise of your mechanic, like I did on the advise of mine. It does not help that you print out 700/900 FAQ, it matters that you read.

that's the thingy near the radiator inlet from the engine. Two hex head cap screws hold the valve to a bracket...gasket underneath the valve. There is a vacuum line from the manifold to the valve. When the engine goes from pressure to vacuum the valve opens and bleeds compressor pressure around the turbo. When the valve leaks, the turbo spins up to try to replace the leak, sucking more air through the amm than gets to the engine, but the ecu reads the amm signal as a demand for beaucoup fuel and increases pulsewidth accordingly, ergo too much fuel and black smoke.

I had to replace my cap screws with hardened socket head cap screws, use lock washers, red loctite, and a new gasket. Has now held for some five years plus.

Sounds like several possibilities.

But if your fuel pressure regulator is leaking, then it's a fire hazard and should be replaced immediately for safety reasons.

Once you've replaced it, you'll learn if the hesitation was in any way related to the failing fpr.

Leaking hoses and a problematic engine temp sensor or connections are also excellent candidates for evaluation. Keep in mind that you should examine the (two-wire) sensor under intake header #3. (The temperature sensor under header #2 is for the temp gauge.)
--
Don Foster (near Cape Cod, MA)

If you have found a leak in the FPR, then replace it with a new one or at least a good used one. If the problem persists but you find no fault with hoses, or air or vacuum leaks, then read 700/900 FAQ particularly regarding engine tune and performace. Systematically test sensors, particularly AMM, connections and grounds at ECU. Before I knew Brickboard, Ringlee and Don Foster, a Volvo mechanic changed the AMM in my 760T three or four times. I paid for two or three of those at a couple of hundred a pop. Still, the car would do as yours, often under acceleration but otherwise unpredictably. I read (studied) 700/900 FAQ and armed with a Fluke and specifications sheets I tested everything in sight and repalced everything out of specifications. The car improved dramatically, but the car still did like yours now and again. Then, I found resistance out of specifications in one of the AMM grounds (over 4 ohms rather than under 1 0hm) at AMM and ECU. Took ECU apart, resoldered and got resistance at both grounds (and other connections) to be 0 ohm. The problem appears to be solved and the car runs better than ever. Knock on wood, steel or whatever.

I believe it's possible for the turbo hoses to have holes in them that aren't visible to the naked eye. What you are describing sounds like the symptoms my car had when there was a tear in one of my hoses. I would depress the gas pedal and the car would feel like something was slowing it down and struggle to make boost. Press the gas pedal in even more and I got a puff of black smoke and a little bit of boost.
--
1988 760Tic - electric fan, HD radiator, boost gauge, remote start, MBC@10psi - 156,800 1966 M-B 230 sedan - 98,000 1982 Rx-7 - 201,000

Another thing to check would be the coolant temp sensor, and the wire to it. It's on the head, under the intake, roughly in the middle of the engine. Busy location, you might actually have to take off the throttle body to access it. If it's open-circuit, the engine will think it's stone cold all the time.
--
Rob Bareiss, New London CT ::: '87 244DL/M47- 221K, 88 744GLE- 202K, 91 244 181K, 88 244GL 145K