I just bought a 1990 740 i/c turbo wagon last weak and it performs poorly under boost. The guage goes almost half way up in the the yellow, but it is down on power for sure. It will barely accelerate at WOT at 80 mph. I changed the oil, spark plugs (looked fine but were platnium), and air filter (was pretty dirty) and the problem persists. There was oil in the intercooler plumbling as well as a few ounces in the bottom of the intercooler itself. I also replaced most of the vacuum lines with new to no avail.

I am going to verify fuel pressure with a gauge and try unbolting the exhaust at the cat to check if it a back pressure problem. Any other ideas?

Thanks,
Shap

Pull the plug on the cat and see if it goes away. It sounds exactly like a clogged cat.
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MD

Good call changing the plugs and the filter. You might also check the AMM/Turbo/Intercooler hoses for any leaks or holes from years of wear. The fact that you can only get halfway up the yellow indicates (in my experience) either your boost is set low or your have a small leak apparent only under higher pressure. Pay particular attention to the output hose from the turbo as the low point can allow oil to pool and rot it out.

A few ounces in the intercooler is a bit excessive. Max you should get when draining the intercooler is 2 tablespoons or so. I would suggest you do all the aforementioned, drain/clean your intercooler, run for a week, and then check the intercooler again. This will tell you if the turbo is throwing lots of oil or the intercooler had never been cleaned.

As for performance, its also possible your exhaust system is in ill repair. Do youy have papers or a work history on this car? When was the cat replaced? Muffler and plumbing?

good luck,
rt

I did take off all the intercooler pipes and inspect and clean them as they were all quite coated with oil. I think most of the oil in the intercooler may have been quite old because it was sludge-like. How do you suggest I clean it out? Perhaps I'll remove it and rinse it with gas

I don't have the service records, so I am in the dark as far as the history. I should be able to determine if its the exhaust or not by disconnecting it at the cat, so I'll do that later today. As far as leaks go, I brake torqued it until it built boost and had a helper listen for leaks under the hood. I'm sure it was making boost because the turbo->ic pipe was ballooning a bit. Nothing was found.

The car just seems to be struggling under boost like something is holding it back.

Shap

Do all the following:

1) Check the performance of the wastegate as indicated in the other post.
2) Disconnect and reconnect the AMM harness from the AMM.
3) Disconnect and reconnect the knock sensor harness from the ks.
4) Check to make sure that the CBV (if this is a garrett) is tight and isn't leaking.
5) Run a cycle or two of STP Fuel System Cleaner through
6) Make sure you are running 91 or greater octane. I actually favor 93, and think anyone that runs anything under 91 is making a mistake. Turbos are high performance vehicles, and higher octanes provide better performance, ESPECIALLY under boost (since a turbo demands higher octane to burn evenly under higher compression). Anyway, under full boost a lower octane will burn unevenly, pre-detonation, etc., so why risk it for $2 a tank.

There's some other stuff you might do, but this (along with the earlier stuff) would be the core items I would check.

good luck,
rt

I have taken off and inspected the connectors for the injectors, KS, and AMM. They all seem to be in good condition, but I made sure anyway. I actually drove with the KS disconnected which didn't seem to make much of a difference except I set an "check engine" light. I was just thinking, the valvetrain is a little noisy, so maybe it is picking that up as false knock? Maybe I'll unbolt the ks from the block and drive it as a test. First I should probably wire the O2 in to the car so I can hook it up to a DMM to make sure I'm not really running lean.

Also, I unbolted my catalytic converter and drove the car to see if back pressure was my restriction. It felt like it made about 15 more hp or so, but still felt bogged (but LOUD). If it was that clogged I think I should have felt an 80 hp difference.

I checked the compression and it was ~130 on all 4, cranking over slowly in very cold weather. The car is actually making boost... about 7 psi w/ my Dave B gauge overlay that I installed last night. I made a 1/4 mile pass to verify the car is actually slow and ran an 18.1 @ 75 mph w/ a 10.x 0-60 and 2.9 60'.

I think the only things left are fuel pressure and ignition. I have a stock Starion coil I might give a try, as well as pull off the cap and rotor to inspect them. Where can I tap my car for fuel pressure?

oh, how I miss OBD-II.... ;)

Shap

You can tap fuel pressure right at the fuel pressure regulator on the rail... that little silver jobber that sticks off the front of the rail and has two hoses on it, onein from the rear or the car and one out to the intake manifold.

See the 700/900 faq for more info on the procedure for testing fuel pressure (center pull down menu at top of this page).

good luck,
rt

I just replaced my turbo...it was using up to 3/4 ltr oil per 100 miles! and nothing would drain from the intercooler!

after replace ment i cleaned the pipes (my favoured mthoed beinga compressor and a parafin was h gun filled with Gunk (engine cleaner) followed by a hose (hint do the engine bay as well...

I tried this method on the intercooler..and it sort of worked .... then i sort of (i.e. did) droped the end off the wash gun into the cooler....ouch..so had to take it out of the car (2 bolts, 2 mins!!!!) and laid it flat and filled it with Gunk (say 1/2 gallon) and sloshed it arround some for a while, then emptied and flushed (carefully) with a hose shiftewd huge amounts of crap....

while it was at it i gunked and flushed the aircon & radiator (again easy with the cooler out) and shifted crud out of both...

2 hours max from lifting screwdriver to feet up and a much sweater car...

I LOVE my compressor, do the engine bay regularly and its so much nicer to work on!

Check the wastegate on the turbo to make sure the cotter pin didn't fall out.