740 Turbo issues

Got a couple of issues that I've noticed recently on my '87 740 Turbo. Not sure if they're related but both are intermittent. Firstly, it has started to idle occasionally at 1200-1400 RPM. I thought it was the idle air valve sticking but I've tried tapping on that and it doesn't seem to make any difference. Next step is to remove, clean, and bench test that.

The other issue is that once the engine is all nice and hot, it starts to stumble when the boost is up near the top. A friend who was following me commented that there was some smoke from the exhaust when this occurred. My first thought was a split in some of the plumbing to the intercooler, it behaves exactly like that except that it doesn't happen when the engine is cold, in fact it may have been going on for some time but I didn't notice it until I was going up a mountain pass on a hot day with the A/C on.

Anyone run into either of these issues? Intermittent problems are tough because they never seem to happen when I'm in any position to diagnose them. Overall the car has been running really well and I got 25-28 mpg on various legs of the recent trip.

Reply to
James Sweet
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Reply to
Jon Robertson

Hi James. I have an '86 740 turbo intercooler. When I first got mine on the road a few years ago, it was during summer, and it ran great except at higher rpms. It would stumble and break up, kind of like a rev limiter when I wound it out. I tried everything I could think of. Finally, I can't remember which, but I either ohmed out the mass air sensor, or swapped in another that I had, and that cured it. It was only doing it when the car was fully hot, or during hot weather. Has been fine since, although at max rpm, it does break down like it has a rev limiter, but I figure that will keep me from blowing it up.

Your mileage! I can only get 22-23 max. I do have the mixture a bit rich, because I'm afraid to lean it too much. How do you set your mixture? Do you follow the procedure found in brickboard using the LED?

good luck, /glenn

Reply to
/g

MAS sounds right. & it does have a limiter\governor.

Reply to
Richard W Langbauer

Use a ohm meter between pins 2 & 6 on the maf sensor adjust the screw very carefully to 700 ohms

Reply to
Glenn K

Thanks Glenn. I just did so. Before adjustment, I had 521 ohms. That's the setting that gave me the 22-23 mpg. I'll see how it does tomorrow to/from work. I'll be filling it up at the end of the day, so we'll see what effect this has.

I do drive the car fairly hard, so that accounts for much of the low mileage. I have a 50 mile commute one way, and run between 75 and 80 mph on the highway.

What else could cause low mileage? Mechanically that is?

One thing that may be affecting it. I believe that at one point in the past, I lost my O2 sensor. Due to lack of money, I put in the one from my '91 940 16 valve. My research led me to believe that the same sensor would work on both cars. Other than that, the car runs great.

thanks much, /glenn

Also, I can't get much more than 6-7 lbs of boost out of it.

Reply to
/g

A better solution is to install a wideband O2 sensor to read the actual mixture, but those can be a bit spendy.

The automatic versions get 18-22 mpg but this has a manual gearbox and consistently gets 22-26 on average, on one occasion I got 29.8 mpg with a light foot on a long fairly flat highway trip.

I'll try a different air mass meter and see if that makes a difference. I'm suspecting something electrical since the problem comes and goes, experience tells me that most other problems come and then just keep getting worse.

Reply to
James Sweet

This was a repeatable puffs of smoke whenever the engine started to stumble, which would occur any time I got up into a significant amount of boost. It felt exactly like it did the time I had a loose hose clamp, except the current symptom only happens when the engine is good and hot. I've meticulously maintained this car so there is little oil accumulation in the intercooler. I clean out all the intake plumbing with Simple Green from time to time.

Reply to
James Sweet

I haven't messed with the mixture at all, the car is stock and until recently always ran great so I never touched it. Unless something is broken, a fuel injection system should never need to be adjusted.

Reply to
James Sweet

How do I know if it is a wideband O2 sensor? Or how do I find one?

Mine has an M46 w/electric OD.

My symptoms came and went. Mine ran perfect in cool weather (it was during summer), but as soon as it got hot out, the problem came back. I do remember coorelating it to a particularly hot day.

I didn't see any smoke in the mirror, but also did not have anyone behind me to check for smoke, so maybe it did puff, but I didn't see it.

thanks, /glenn

Reply to
/g

If you're not sure, then you don't have a wideband O2 sensor. They were not OEM equipment until the mid 2000's or so, and even today many cars still use narrowband sensors. Aftermarket wideband systems are available and useful for diagnostics & tuning, but rarely really needed.

I couldn't see any smoke in the mirror either, it was only when I had someone behind me that I found out. At any rate I swapped the air mass meter tonight and it seems to be running noticeably more smoothly and with a bit more power. I noticed when I got back that the main fuel pump (which I replaced a couple years ago) was noisy and I can't hear the in-tank pump running so I ordered a replacement for that. Fingers crossed that this fixes any remaining issues.

Reply to
James Sweet

Reply to
Jon Robertson

That's the next step, I've got a cleaned up spare that I'll pop in and see if it fixes the fast idle. I swapped the air mass meter and it has been running noticeably better, feels more powerful and smoother although I haven't had the combination of a hot day and mountain pass to really test it.

The pre-pump is shot as well, I can hear the main pump (which I replaced last year) cavitating and no nose from the tank, replacement is on order and will be installed shortly.

Changed some vacuum lines too that were getting ratty, they hadn't split yet but can't hurt to have nice new ones in there.

Reply to
James Sweet

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