Only getting 1.2 on boost gauge
 

I am having a boost problem with my 89 951. It is a stock 951 and I use premium pump gas. Up until now it has run great, including track days.
I have looked for threads with a similar problem, but none seem the same, so I am starting this thread.
When I first start the car I can usually get the boost to up to the normal ~1.8 one time, from then on it will only to 1.2 bar. If while driving I shut the engine off and then restart it I can get it to go to full boost one time then only to 1.2 bar.
I connected a small 12v light to the wires going to the cycling valve, when I can get it to go to full boost the light comes on as the cycling valve opens. After the first boost it only goes to 1.2 bar and the light does not light up. The waste gate seems to be working ok. Cold at idle, if I give it some pressure it opens up and the downstream pipe gets hot. I put an accuboost on and it will boost past 1.2. This seems odd, if the cycling valve doesn't open why does the accuboost change anything.
It also seems to have more turbo lag than it used to. The A/F ratio is ok around 15 normal driving and about 11.5 at the 1.2 bars. I have checked for exhaust leaks and did not find any. I put in a set of cooler plugs, NGK BPR7ES, thinking it might be a detonation problem, but that made no difference. I have looked at the turbo it is in good shape, no damage to the compressor, little end or radial play. The throttle positon sensor seems ok, resistance on both pins is fairly smooth and the throttle open switch looks good. I have tried swaping the AMF for a MAF sensor, still the same, A/F ratio still ok.
I have taken the accuboost and the MAF off, I will put them on when I get the problem sorted.
Any one have any thoughts could it be the DME or KLR trying to protect the engine for some reason? Could it be knock? I have tried every thing I can think of.
Thanks in advance.

Sounds like the car is going into overboost protection mode. The cycling valve may be starting to fail. Try swapping it with a known good one to see if that changes anything.

what chips do you have in it? if you are putting a MAF and a accuboost on it, I would have thought the CV would have been bypassed.

and how does the diverter valve look? ripped diaphragm? any flutter sound when you let off after boost?

after it hits 1.8 then 1.2, park it, leave it running and check for blink codes using a simple LED in the 12v test port. http://members.rennlist.com/951_racerx/
click "test fault codes". quick and easy

1.2 bar is the ‘limp mode’ setting. The DME is seeing a signal that it does not like – limp mode is trigger by a lot of different possibilities. If it were overboost then you would get a sudden and violent engine shut down while at full boost. Its not knock either – that would result in a reduction in boost until the knock is not detected – not full limp mode.

By your description the rest of the system is working as designed. The boost control system works by letting the wastegate see boost pressure – if the wastegate line transmits the pressure from the intake manifold the max boost you should see is 1.2. the CV (and the accuboost) regulate the pressure signal to the wastegatge allowing for more boost.

+3.........

I had a similar problem with my '86 turbo, except it would last about an hour before going back to limp mode. At the time the blink code suggested TPS was faulty, but I thoroughly checked it and could find no fault. Continued checking other stuff for about 6 monthsr then gave up & replaced the TPS - no more problems.
You'll definitely want to set up an led, preferably inside the cabin, to check the blink code then go from there. Remember you need to leave the engine running to check the code.

I had a similar problem with an 87 951. Boost would drop off as described after running the car for awhile. It would reset after shutting off the engine and restarting Mine has a 3" exhaust and Lindsey Wastegate. Otherwise it's completely stock. The only thing that cured it was installing an aftermarket DME chip. That stopped it from going into the limp mode setting.

Thank you all for the help, I tried to find the blink codes for the 89 turbo but I never found anything, just for the 968. joster thank you for the link to Racer X's blink code page. I set up a led and got a 2-2 code, "Knock Sensor supplies excessively small or no signal, or K/CP control unit is defective". I turns out that the wires just before the connector were cracked and I could see corroded copper wires. I don't know if the wires were touching or not. I spliced in another connector, put everything back together and that did the trick, it runs great again.

To answer a few of the questions. The blow off valve is in good condition, I have recently replaced it. There is an Auto Authority chip in the DME but not the KLR, that is the way I bought the car.

I want to put the MAF on for a couple of reasons. My stock filter housing is held together with 3 strung together hose clamps, I am afraid of the AMF going bad, and to help get rid of some turbo lag. I am putting on the Accuboost, set to 5psi, just to help with the turbo lag.

i had a 1.2 boost issue as well and it was because i forgot to plug the Throttle Position Switch back in after putting it back together. that may not be it but that was an easy fix.

make sure you check all your wires because there are probably half a dozen or so more that are in similar condition