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Question high boost hesitation, missing, intermittant

I have a late production 1985 Porsche 951. It has 79,900 miles and has been well cared for. It has two modifications; sport exhaust and chip. I use only 93 or better octane fuel.

When I go to wide open throttle, the boost gauge climbs and everything goes as expected until the boost gauge almost tops out (just under 2 bar indicated). At this point the engine seems to miss and jutters almost like it is hitting the rev limiter. It is sometimes quite violent. The engine recovers instantly as soon as the boost drops a little. Just backing off the accelerator smoothes it right out.This does not happen if the boost is down and the engine will rev to redline without any problems. At first, I thought it might be related to humidity but I drove the car tonight (relatively low humidity) and it displayed the problem. This problem occurs more often than not recently, but there are occasions where is does not occur at all.

Nothing has changed recently and I can't correlate the start of the problem with any other event. Oil and temperature gauges read as expected.

Any help you provide will be greatly appreciated.

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Old 06-19-2003, 03:27 AM
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The first place I would look is the vacuum system. Those little tubes that go all over your engine top age and crack especially at tees. There are at least two tees where three vacuum lines come together. One is by the fuel damper, another between #3 and #4 intake runners. If they are cracked, the boost will not work properly. There may be another tee near the back of the engine. If the tees are cracked, the engine won't run properly.

Another area that I would check is the circular valve that is next to your coolant reservoir. Its purpose to let compressed air back into the intake of the compressor so you don't lose turbo rpms if you let up on the gas. The valve is spring loaded and MUST remain closed unless a vacuum is applied to the nipple at the top of the valve. If it opens, your engine immediately bogs down. To check the valve, remove it and try to blow through one of the large holes. No air should pass. Apply vacuum to the nipple at the valve top. The valve should open. There should be absolutely no leakage of vacuum into the part of the valve where the positive pressure exits. None, nada, zip!

Another possibility is a crack in the big hose coming off the compressor part of the turbo. The elbow shaped hose hardens with age and might work until just enough boost is present to suddenly open the crack. Also check the rubber unions between boost tubes and intercooler and boost tube and throttle body. Also check for cracks in the short hoses where the circular valve attaches to the boost tube and line going to turbo intake. This is valve I was talking about above. I actually had my valve get loose. It would work ok for low boost but would let air escape at high boost bogging engine.

Lastly, the recycling valve between #3 and #4 intake has three plastic nipples on it. Feel around in there to see if the nipples are still there with the hoses attached.
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Lawrence

1986 951
2002 SLK32 AMG
1987 328GTS
2005 TL

Last edited by Lawrence Coppari; 06-19-2003 at 04:00 AM..
Old 06-19-2003, 03:57 AM
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First...I would be afraid your funning too lean...does this not sound lean to anyone else? I mean lean as in not enough fuel pressure. This can be VERY bad! Ahhh! It is chipped? Perhaps your fuel pump is on its way out? perhaps fuel regulator not working? perhaps you need a 3 bar reg. At anyrate...don't burn your motor! Figure out what it is!
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Old 06-20-2003, 08:47 PM
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Quote:
Originally posted by Lawrence Coppari
Lastly, the recycling valve between #3 and #4 intake has three plastic nipples on it. Feel around in there to see if the nipples are still there with the hoses attached.
Actually, this is the first thing I would check. If the cycling valve goes bad, it won't send pressure to open the wastegate and you'll get an overboost condition, and the computer will shut off fuel -- this feels just like hitting the rev limiter as you described.
Old 06-21-2003, 03:17 AM
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Excellent tips, Gents. I obviously came to the right place. I am currently checking the vacuum lines, the recycling valve, and the circular valve next to the coolant res. I will also take a look at the plugs to see if I am too lean. Is there any better way to check if the motor is running too lean?

Thanks a bunch!!!
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At first inspection, all the vacuum lines appear ok and so do the T's. I removed the circular vavle by the coolant res and I think it is bad. I can not blow through the bottom of the valve but if I apply vacuum I get leakage on the side that goes into the intake. Is this what you where saying indicated a faulty valve?
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Old 06-21-2003, 04:08 PM
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Yes, the valve is bad. There is a leak in its diaphragm. The nipple should not connect to anything. All it does is open the valve against a spring. It is letting air into your vacuum system which controls a number of things.
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1986 951
2002 SLK32 AMG
1987 328GTS
2005 TL
Old 06-22-2003, 10:57 AM
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Quote:
When I go to wide open throttle, the boost gauge climbs and everything goes as expected until the boost gauge almost tops out (just under 2 bar indicated). At this point the engine seems to miss and jutters almost like it is hitting the rev limiter. It is sometimes quite violent. The engine recovers instantly as soon as the boost drops a little. Just backing off the accelerator smoothes it right out.This does not happen if the boost is down and the engine will rev to redline without any problems.
There are two things that can cause this, overboost protection or detonation. You mention a chip, most aftermarket chips (2 for the turbo) have the overboost protection disabled in the KLR chip. Do you know what brand of chip(s) you have? I have seen somebody that sell just a DME chip and thus the overboost protection would still kick in at about 2 bar absolute. if this is your problem, I would check out the vacuum lines running from the intercooler tube to the cycling valve (round brown valve located under the manifold between the #3 & #4 runners) and the final line going down to the wastegate.

The second possibility, detonation, could be caused by running lean, a faulty or weak injector, or carbon buildup.

Quote:
I removed the circular vavle by the coolant res and I think it is bad. I can not blow through the bottom of the valve but if I apply vacuum I get leakage on the side that goes into the intake.
If you can pull and hold a vacuum on the top nipple, the recirc valve is good.

Quote:
The nipple should not connect to anything.
I don't understand what you mean by this. The nipple on the top of the recirc valve gets conneted to the intake manifold (normally tied into the line by the fuel line pressure damper). This way, pressure is applied to the diampram to hold the valve closed under boost and to supply a vacuum signal to open the valve between shifts.

Rob
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Last edited by robm-951; 06-22-2003 at 08:28 PM..
Old 06-22-2003, 08:25 PM
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There must be NO leakage at the nipple. It connects to a diaphragm. If you apply a vacuum, the vacuum should remain. If you apply pressure, the pressure should remain. It does not directly connect to any other portion of the valve. If you apply a vacuum to it, and the vacuum goes away, then you have a leak probably in the diaphragm. If it does not hold vacuum or positive pressure, get a new one.

This is the point I was making.
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1986 951
2002 SLK32 AMG
1987 328GTS
2005 TL
Old 06-23-2003, 03:03 AM
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Thanks Guys! As per Lawrence's last few posts, I have established that the diaphragm on the valve by the coolant is definitely defective. I have a new one on order and will test as soon as I get it installed. Call me dippy, but I can't find the cycling valve. Is it hidden under the intake manifold or am I missing the obvious? I know the chips came from Autothority but I can't remember if there were one or two. I am currently looking for the old parts as a comparison.
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Old 06-23-2003, 03:09 PM
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Quote:
This is the point I was making.
OK Lawrence, now I understand. I figured you knew what you were talking about, but the point behind the sentence just didn't click for me.


Quote:
Call me dippy, but I can't find the cycling valve. Is it hidden under the intake manifold or am I missing the obvious?
It's not really obvious, I have attached a picture to help out. The manual refers to the cycling valve as a timing valve.

Rob

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Old 06-23-2003, 05:49 PM
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Originally posted by mad_german
I know the chips came from Autothority but I can't remember if there were one or two. I am currently looking for the old parts as a comparison.
I just checked and it is Autothority that sells the single chip upgrade (DME only) as well as the 2 chip upgrade (DME & KLR).

You can check out the 2 kits at the Autothority website

Rob
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Old 06-23-2003, 05:58 PM
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It can all so be your over boost protection. Check the line that goes from the cv to the waste gate.

I have just replaced my Air circulation valve and I'm still losing boost pressure.
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Old 06-23-2003, 09:05 PM
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Uhh.. the Stage-1 APE upgrade is the single-chip DME-only upgrade. It is not meant to be used with increased boost !!! That means the boost-gauge should only go up to 1.8bar and no more. Above this level, the stock KLR chip will activate the overboost-protection.

Most likely the culprit is a leak someplace in the cycling-valve hoses. This would cause insufficient pressure to make it to the wastegate to regulate boost, thus higher-than-stock boost.
Old 07-12-2003, 12:04 AM
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Or... if you have the Stage-2 APE chip upgrade, then overboost-protection shouldn't be an issue. In which case, this clue: " It is sometimes quite violent." would seem to indicate a leak big enough to severely mess up air-fuel ratios. Such as if enouigh air leaked out that the mixture was so rich it causes the stumbling. The only places that can leak this much air is the turob-outlet hose and the intercooler-pipe hose-connections... Good luck!
Old 07-12-2003, 12:11 AM
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Cool

It is a stage two kit and the problem seems to be solved! I replaced the valve that keeps the turbo spooled up (by the coolant res) and the boost was a lot smoother but it still had signs of the problem. I then put a new set of wires on. While replacing the wires I noticed that there was a bunch of slop in the dist. cap. I reseated the cap and the problem is GONE. I would never have pegged this as an ignition problem. This is really screwy as I don't understand why the motor dist. cap would only cause the problem when the turbo spooled up. Maybe it was throwing the advance off??? This problem still has me confused but at least it hasn't come back yet!
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Old 07-12-2003, 04:02 AM
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yeah, strange too. It would appear a loose distributor cap would cause you problems regardless of boost. Perhaps it was RPM-related and you only had high-RPM with high-boost? I've pulled one of these off a car that was performing perfectly (the one on the right):



We actually only discovered that disttributor rotor a month after running this dyno chart.
Old 07-12-2003, 08:25 AM
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