My 86 745T stubles/stalls for short (1-2 second) periods, but never quits running. During "between stall intervals" the engine runs fine. These incidents are almost constant, occuring randomly but frequently (intervals between range from 2-10 seconds). It's worse on acceleration, when it feels as if the timing retards suddenly. It runs like this all the time. It also requires longer periods of cranking after sitting overnight or on an incline(i.e. driveway) which seems like it simply has to wait to get fuel (newer symptom; odd for an electric fuel pump design).

Since this began, I've done plugs/wires/rotor/cap, replaced the fuel filter and the in-tank fuel pump. I did a fuel pressure test (remained fairly constant at about 36 PSI and double-checked all the vacuum hoses (all are less than 3 years old). Interesting side note, this problem first cropped up 2 months ago--that's when I did the tune-up/pre-pump/filter/pressure testing. It stopped the stumble, but lacked pep and continued to get 15 mpg (down from 20 or so). Now the stumble has returned.

The Haynes manual says the FT is supposed to maintain 42psi, but the F is supposed to maintain 36psi. Could the previous owner have installed the wrong pump? If so, would this explain my symptoms? Is is possible for a fuel pump to "get tired"? I really don't want to buy the pump unless I'm sure it's the problem.

Any other ideas?

Have you done a timing belt lately?

One guy I know had his done by a second-rate mechanic who who aligned the cam sproket wrong, put the thing back together, retrded the timing so much the vehicle ran poorly for weeks until the owner finally opened the thing up and realized the mechanic hadn't bothered to ensure the No. 1 piston had to be tdc to line up with the woodruf key yaddity yaddity.

Other rare occurrances are the belt is so loose and worn it can "jump" a sprocket, causing all kinds of weird and wonderful things...

David, I have just experieced the same problem with my 86 740T. I replaced the FPR and no further problems. A properly performing FPR runs at 43psi, and this is critical for good performance. Your FPR delivering only 36psi is the answer to your problem. Anyway all the best.

Craig Jordan

Thanks for the suggestions, folks. I replaced the FPR; no change in performance. Replaced main fuel pump, no change in performance. Groan. So now both pumps, the filter, and the FPR have been replaced with absolutely no change in performance. HELP!

4 weeks ago, when I took a Saturday to do the diagnostics (fuel pressure etc.) the bucking problem went away, although the mileage stayed around 15mpg and the acceleration was a bit anemic. But the day after my son totaled my 535i the bucking started again on the Volvo. (Bad luck comes in threes, I'm still waiting for #3) Any additional advice would be welcome --- could it be an ignition problem? Yes, I've read thru Steve Ringlee's faqs, can't find anything else that fully describes my problem -- although some have similar characteristics. Note that the tach needle drops a little then jumps back each time it bucks. (All sensors were replaced 2 yrs and 20,000 miles ago along with the engine wiring harness) Could it be a disty problem (O-rings replaced with the sensors/harness, but now there is some oil residue appearing)?

First, check the fuel pressure regulator. I removed the vacuum hose from the manifold, and sucked on the hose. Fuel taste told me to replace FPR ($50 or so).

Also, check your O2 sensor (700/900 FAQ has procedure). I replaced mine, and my bucking/hesitation under throttle went away. My gas consumption decreased as well.

Finally, my mechanic has told me that Bosch platinum plugs are no good for a Volvo turbo engine. He recommended Bosch Super (copper core) one heat range hotter than spec. This also helped.

Good luck. Study the 700/900 FAQ.

Also, it's the fuel pressure regulator that determines the pressure, not the pumps. O2 sensor had me bucking for a while too.

Rene