Today we tried a replacment AFM and that had no result. We tried a newish FPR with no result. We also took a look at the timing marks on the bell housing and in the distro inspection cap and verified that they were aligned to TDC. Using a homemade test indicator, we were unable to read any error codes from the DME/KLR. Throttle position sensor checks out. Not sure what to check next.

 

Gene,

Sorry that the FPR and my AFM didn't fix your stumbling problem.

The hesitation in your throttle response seems to occur at only partial throttle as far as I could tell. I was also surprised to see it happen without a load on the engine. I wonder if there is a part of the intake that would cause such a hesitation if it was not running or installed correctly?

Does anyone else have any suggestions as to what the problem might be?

BTW Gene, if you need ANY assistance, part swaps or tools let me know. I'd be more than happy to lend a hand.

 

The fuel rail and manifold are coming off tonight. I will take a peek under there to see if there is any leaks. I am also thinking of replacing the O2 sensor as well.

Thanks for the offer of help. Hopefully the manifold will cooperate and come off. I pulled the fuel rail and it looks like one of the allen head bolts may be stripped. That will be no fun.

 

What exactly happens? Overboost protection will kick in it will feel like someone turned the key off although usually the car will still run it can also happen at part throttle anytime your making boost if you made the code tool you have to check it as soon as it happens, once you turn the car off completely it resets, hence no codes if you do a search you will find plenty of info on this. Hope this helps.

 

I initially thought it was overboost. When it happens, the engine stumbles badly. I can reproduce the problem over and over and do not have to turn the engine off and then back on as some others have described. Also, when I just slam the throttle open to WOT at any speed, there is a noticable lag. This is very obvious when the car is idling. I open the throttle and the engine lags for a moment and sometimes there is a clanking sound from the AFM as the gate slams closed.

We were unable to read a blink code after the making the problem happen.

 

First off, this started immediately after you did some work on your car, so the two things are likely related.

If it only happens when you get on the boost, it might be a slit or crack in one of the large rubber boots or hoses, possibly undernmeath where you can't see it . The slit opens up under pressure then closes again, it's fairly common. Did you have to remove any of them when you were doing the other work on your car? I doubt it's under your manifold.

What stripped on the allen screws, the head opf the screw? If so, were you using the correct size key? Get the right one before proceeding. Allen screws rarely strip. If anything did strip, it's likely the head, that will need to have a helicoil inserted.

 

That definitely sounds like a airleak somewhere. I'd have to agree with peckster start at the a/f meter and just keep checking your connections make sure you don't have any broken wires anywhere that happened to me once.

 

The allen head bolt on the manifold was pre-stripped. I have yet to loosen it, but I can see that it is damaged just by looking.

I removed none of the hoses when I worked on the car. The only thing that was removed was the airbox. It is conceivable the airbox removal may have damaged the j-pipe. I will examine it now.

Thanks!

 

The J-pipe has to be soft, otherwise it will crack when you work on your car. Buying a new one is a good investment for sure.

 

I was thinking since the KLR doesn't read any codes, that the boost was maybe fluctuating, i guess the leak in one of the rubber parts would support that....

 

The j-pipe is nice and soft and defect free. It must have been replaced at some point. The tubing underneath the manifold is a disaster area. First, the idle stablizer had broken free of its mount and was free to rattle around. I can't determine if it was replaced at one point and not installed properly or if the rubber mounting bolts had broken free. Do the mounts have a nut on each end and a rubber bushing between them? I will probably fabricate replacement mounting hardware rather than buying some obscure Porsche part.

Second, the plastic venturi was cracked and the inner part of the venturi was sliding in and out of the main body. This part looks like a real piece of junk. I have heard that people have removed it from their system. The hoses connecting to the venturi were damaged and the hose clamps had erdoded through the tubing. Some enterprising mechanic had used safety wire and twisted it around the various hosing ends in an attempt to pull all of the hoses onto the venturi. It was pretty funny to look at. New hoses and clamps are definitely in order. It is shocking to think someone would go to the trouble to remove the manifold and then not bother with replacing worn hoses and broken hose clamps.

I am wondering if I should replace the cyclic valve just as a safety precaution while I am in there? Is there a way to test it out of the car?

Also, the wiring harness has a few cracks in the insulation. The wiring in general is brittle from the hear. Is there a type of tape that can handle the heat? Shrinkwrap tubing isn't an option as far as I can tell as there is no open end to slide the tubing on.

It is unknown if cleaning this up will help with my problem, but it will be reassuring to know that there are no lurking issues under the manifold.

 

The problem has been resolved. Removing the venturi tube nightmare fixed the problem. I now have a simple system that is present in the 88 and newer cars. A tube from the manifold to the brake booster and another tube from the manifold directly to the idle stabilizer.

Oh the joy of throwing the safety wired, broken and cracked venturi assembly into the trash. It was quite liberating.

I also resealed all of the injectors while the fuel rail was off. The old seals and hats were pretty bleak. The kit was $2 per injector at the local Kragen. What a deal.

There is no more hesistation of stumbling when going WOT in neutral. Going WOT in gear results in steady boost. The cyclic jerking is gone.
The stock boost gauge no longer goes up to 2 and I know longer feel to same amount of boost that I did before. I am not too concerned right now, because I would rather underboost than overboost. I did manage to lose both of the copper washers that go on either side of the banjo bolt that connects to the intercooler pipe. Perhaps there is a boost leak there now.

At least the engine response is now predictable and the nagging mystery of what is under the intake manifold is solved.

Thanks for all of your help!

Gene

 

Not sure what the venturi is or what it does. Where is it?

Glad you found the fix.

 

Check out the discussion of the venturi tube on this thread:

https://rennlist.com/forums/showthre...threadid=99044

Basically, on pre-88 cars, under the instake manifold, there is a metal y connector, a plastic venturi tube with three ports and a metal flow restrictor. This whole apparatus is nested around the idle stabilizer and
is begging to leak. On my car, the venturi was cracked and broken and the hoses were worn. They we also safety wired onto the venturi and y-tube. (!)

Read the above thread for more discussion on this. It is a very complex system that seems to deliver no benefit. It felt good to remove it.

Gene