760T Poor idle, fluttery bypass valve and stumble

I have a 1990 760T. I put in a newer motor with ~120k about 10k ago, in the process upgrading new seals bearings, rings, porting the heads and porting/rebuilding the turbo.

I'm having trouble diagnosing whats going on, my car has had a poor idle for the last 20k miles or so, I have tried to pressure test the intake side with soap on all the joints, but found no leaks, it will waver up and down between 450-600 rpm causing a lot of vibration when it goes below 500.... its extremely annoying.

I installed a V-Cam, but this didn't help or hurt the problem...

Originally the car had EGR system and 5th injector, which I removed because I used the intake & exh manifold that came with the newer motor which didnt have provisions for EGR or 5th injector. Could removing these throw the ECU off and cause poor idle?

Could it be MAF Sensor? I've used MAF cleaner on it....

I have considered just getting the Ostrich ECU tuner and turning up the idle, but i'd rather find the root cause of the problem rather than use a band-aid.

Any help?

Recently another issue has cropped up, which I think is unrelated. If I get into boost and let off the throttle, the bypass valve now sounds like an aftermarket blow off valve, with a flutter noise that lasts 2-3 seconds vs. the 1 second whoosh noise it used to make. Immediately after letting off the throttle the car will drop a couple hundred RPM. If I build boost and come to an quick stop afterwards the car will die... Really sucks in stop and go traffic. Any thoughts on that?

Could be a bad/leaking CBV. But you do say flutter? As in compressor surge? As in the CBV isn't functioning? Have you checked the vacuum lines to the CBV?

Asfor your idle issue, is the engine completely normal otherwise? You're idling between 450 and 600. Have you changed your idle air control valve yet? If you elevate the idle to ~800rpm with your foot, does it behave properly?

ziddey said:

Could be a bad/leaking CBV. But you do say flutter? As in compressor surge? As in the CBV isn't functioning? Have you checked the vacuum lines to the CBV?

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Its a flutter noise, surge happens when trying to boost to high psi too early in RPM right? This problem happens after I boost and let off the throttle, where it would normally make a quick woosh noise, now it makes a slower fluttery noise. It seems like the bypass valve just isn't opening? I will check the lines and pop the cover off the bypass valve tomorrow as see whats up.

ziddey said:

Asfor your idle issue, is the engine completely normal otherwise? You're idling between 450 and 600. Have you changed your idle air control valve yet? If you elevate the idle to ~800rpm with your foot, does it behave properly?

Click to expand...

Yeah the engines is just fine otherwise, It will idle at about 1000 RPM until warm and the problem is the worst when fully warm, it will hunt around 500-600. I tested a different IAC valve with no change, I also cleaned them both out with no change. I tried unplugging them and the car wacked out and died. When I elevate the idle to ~800 the idle still hunts, but now it will hunt between ~700-900, it does this at all RPM. Here is a quick Video of the idle, and then what happens when I boost and let off the gas.

http://www.youtube.com/watch?v=Q-bYIBtqUCw

Note, in the vid the car isn't fully warm yet, which means the idle normally is lower.... The loud rattles are just trim peices rattling.

Today I started off by checking the Turbo Bypass Valve - I should note that when I first installed it I was aware of a tiny tear, which I superglued to keep from getting worse. Today, it looks like the superglue is holding pretty well and although the CBV isn't pretty I cant see more than the tiniest of places where enough air could be escaping to cause this problem.

Then I did a compression test today, for cylinders 1,2,3,4 the results are 157, 158, 148, 151 so I know that all cylinders are easily within 10%.

Afterwards, I started the car and now cylinder #1 is being tempermental. It will only fire at 1500 RPM and upwards with no load, or 2000 RPM or more with load, but will cut out again if I go above 4 PSI of boost. While in gear its seems like the more load the worse it gets, but with no load it will only occur <1500 RPM

I double checked all ignition component connections, then I narrowed the problem to cylinder 1 by pulling the injector wire connector. When pulling #1 nothing happens, when pulling any of the others I go down to 2 cylinders. Continuing to troubleshoot I pulled the #1 spark plug and it looks the same as the rest, no fouling. Then I tested the resistance on the wire which measured at 5.7k Ohms and noted that 5k Ohms is written on the connector, so that should be fine too.

Thoughts?

Fluttering on liftoff IS compressor surge, in this case caused by the CBV not opening, resulting in boost having nowhere to go since the TB is shut. Triple check that the vacuum line to the CBV is A-OK. If you're positive it is, apply vacuum to the CBV and see if the valve opens.

As for your ignition problem, if the other plugs are firing properly, suspect the wire to #1 or the #1 plug. It is possible but extremely unlikely for your #1 point on your cap to be burnt out, but if it comes to it, give it a check. First things first, swap wires between say #1 and #2 and see what happens. Then get a new plug for #1.

Some of what is described sounds like when my intake mani gasket was bad at No.1. I see leaks were checked for, but...

isolated the miss in cylinder #1 to the wire. This is the second set of IPD wires to fail on me... Luckily I had the old set laying around so I swapped the number 1 wire from the "old" set in and all is well. The resistance in the problem wire was 5.7k close to the 5k it should be, but it was still faulty. The resistance in the replacement I put in was around 500k ohm which is 100x what its supposed to be... its just wierd and I dont understand but it fixed the prob so im not gonna bitch.

Thanks for the learning on compressor surge, that makes sense when you explain that the air has no where to go. I will check all the vacuum lines again, in theory the BPV vacuum line should go to the intake manifold correct? I wonder if I could T it into my boost gauge to see if I can visualize what its doing.

Jerry, thanks for the tip on the intake gasket, I really haven't done a proper vacuum leak test, though I did attempt a pressure test. I couldnt get system properly pressurized, what would be a good recommendation for where and how to get it pressurized?

Thanks for the help thus far.

Figure out the diameter of the intake pipe on top of the airbox. Home depot should have a rubber cap that'll fit (plumbing section I believe). Drill a little hole to put your nipple or whatever and hook that up to a handpump/bikepump/whathaveyou. Problem is it may be hard to see/hear where the leak is, which is why shops tend to smoketest it.