deranged

I have a 1986 928 with 60k miles on it. I have done all the necessary repairs and replacement of
parts to get the vehicle running again; vacuum lines, tune up, water pump,
timing belt, trans. service, etc.

When I was doing the top end refresh, I removed the throttle, bowden, cruise
and pedal cables from the quadrant. I think when I reconnected them I lost
some adjustments. I reviewed some of the forums to find some solutions to my
problem of hesitation in upshifting at high rpm.

The vehicle starts fine, and idles at about 750-800. The trans. fluid level is
normal and recently replaced. There is vacuum at the brake booster. If I
accelerate normally, the vehicle shifts perfectly. However, when I push the
pedal to the floor, it starts hesitating and does not want to upshift. If I
back off the throttle, it will shift. If I do not aggressively accelerate
the vehicle shifts perfectly. If I am at 60 mph, and press the pedal to the
floor, it does not kickdown, but sputters, hesitates as if it wants to shift
to a lower gear. I will only shift if I back off the throttle. I have
adjusted and readjusted the bowden and throttle and pedal cables dozens of
times with no correction. Interestingly, it seems to run find, even at full
throttle, when it cold. I may be mistaken, but I think these symptoms occur
only when the vehicle is at operating temperature.

On a 30 mile trip recently it does not want to upshift on hard acceleration, but bucks and hesitates. I can drive in the morning for 30 minutes with no problems with acceleration, kickdown at 55 and so forth.

Then I park it for 5-10 minutes. It always starts right up.

Then take off again and the symptoms of hesitation in upshift, bucking occur, and will not upshift unless I get off the accelerator. This hesitation/bucking occurs on hard
acceleration.

It seems to me heat is the culprit. I have checked the
vacuum line at brake booster which checks with good vacuum. I have not checked the line going to the modulator on the tranny.

Would a vacuum leak occur only after the vehicle is hot?

I thought I would try manually shifting to see what
happens.

So I put it in 2nd and when I get up to 20mph it stars bucking,
like it does not want to go faster than 20. I did not see what would happen
if I manually shifted to 30. But when the shift lever is put in 2, and I
drive, it will not go above 20 and then starts this bucking that I
experience when it automatically shifts.

So is driving in 2 and not going above 20 normal?

86 928 automatic

RFJ

NOT normal at all,but all i have are 5 speeds wait till one of the experts chime in I am sure you will be rewarded. best Ray you might also search this forum as there is much to be found.

deranged

Been all over it for weeks. Thanks for your input.

SMTCapeCod

Note- this is mainly a friendly 'bump to the top', but as long as I'm here.....WAGs:

Sounds to me like WOT switch is at least a contributing culprit. IIRC the WOT signal informs trans shift pattern. I don't think it is seeing that you are at WOT, and it may not be seeing that you are off idle. Can be tested via the plug by the diagnostic port near the oil neck....if your plug/harness aren't disintegrated...

Is there any chance that some of the vacuum lines were misrouted on reassembly? In some cases the main line of the little plastic manifold aft of the MAF that provides vacuum back to the modulator has been found off...usually entails very hard shifts though. Modulators themselves have been found to leak- I don't know whether the leak correlates with Hg when it actuates, I assumed it was a steady leak, but that could be a contributing factor. Also measuring the actual vacuum at the line that goes into the computer may be a good idea.

I believe that there is a Hi temp switch/circuit in the valve body...but I have no idea what it does...

GreGG??? :-)

soontobered84

Is your MAF connection good and secure? Can you swap MAF's with somebody close?

Mrmerlin

did you replace the short harness for the TPS? the connector breaks
have you verified that both coil wires are not touching any part of their run,
and looked for sparks while running at night?

deranged

MAF was replaced in the rebuild. About 100 miles on it.

deranged

Coil wires, ignition wires all new......

Short harness for TPS....I do not think I replaced it, however, after removing the throttle body and cleaning it, I do not recall seeing any connectors broken.

deranged

All the new vacuum lines were replaced one at a time, removing one, installing a new one, etc.

Have a PM into Steve C. hoping he can shed some light on this.

Thanks, everyone for your brainstorming on this. Much appreciated.

deranged

Thinking my shift issue is heat related, tonight, just now, I took it for a spin. It is cold here, 50 degrees Fahrenheit. I did everything I normally do, driving it like it is stolen. So
what happens? Now it runs like the Batmobile again. NO HESITATION, and over
over 80 mph before I get 3rd gear. I can even manual shift and not get ANY
hesitation in any gear.

I have a feeling the hesitation is heat related....which leads me to think MAF?

If cold air is blowing into the MAF cooling that wire constantly, would this
affect the performance? Because I drove tonight long after I reached
operating temp. and it never hesitated.

So why when the engine is colder than operating temp. and why when the
outside temp. is colder does it run like it should?

I used to overhaul aircraft engines. We always said the turbines and
compressors loved the cold winter air passing through them and the engines
always ran cooler and better.

Any thougth on this?


928 automatic '86

SMTCapeCod

Subscribed, good mystery have experienced some similar manifestations.

SMTCapeCod

Installed a rebuilt valve body.
Remarkable difference:

1st gear start restored, responsive part throttle downshifting restored, and most importantly the response to throttle imputs is no longer slushy. I spent a lot of time adjusting cables, chasing assumed vacuum leaks, etc. The valve body was indicated by some of the factory diagnostic decision trees...and I guess they new best..

deranged

Would like to post this for others who have or had a similar issue of the one I previously described. The solution was found after I discovered from 928Motorsport's website the following quote:

"A faulty crank reference sensor can completely disable your car, or shut it off just when the engine is hot."

Replacing the crank sensor with a new one solved the problem. Vehicle now runs like it was born to run.

________________
928 Automatic 86

deranged

On another issue, I purchased the VW interior light fixtures from Roger Tyson and wish to replace them as the originals have fallen apart. My question: is anyone familiar with these fixtures and has anyone installed these fixtures in their vehicle?

There are several wires connected to the original fixture as shown in the photo above. Roger's fixtures contain three yellow wires. I am not exactly sure how these three yellow wires are to be wired up to the new fixtures.

The new fixture has a three wire connection on one side of the fixture for each of the yellow wires Roger provides in his conversion kit. He does not include a schematic for the replacement.

Some insight would be greatly appreciated. Thanks.

________________
928 Automatic 86

Bill Ball

Unusual solution for hesitation and shifting issues. A bad crank sensor disables the car completely, perhaps only when hot. No spark, no fuel, no start.