964 with Engine bucking bronco problem

[America Roadste]

Has anyone experienced a problem with their 964 having engine bucking issues when cold. I live at 3100 ft and when I come down the mountain I am not gassing it. When I get on flat land, the car will buck as if it is trying to get enough fuel to run correctly. Often it dies and when so, I shut of and crank up and it sometimes runs correctly and sometimes it runs a short distance and does the same thing again. After several complete shutdowns and start ups, it runs great. I replaced the DME relay and fuel pump based on the Porsche mechanics suggestions. The car has 116,000 miles and is well maintained. Someone else suggested I try replacing the cylinder head temperature sensor. Before I go this route, I would like additional advice because I but do not want replace components as we search for the problem.

[Traveller]

Originally Posted by America Roadste

Someone else suggested I try replacing the cylinder head temperature sensor. Before I go this route, I would like additional advice because I but do not want replace components as we search for the problem.

According to the Porsche wiring diagram, the one on my 993 should read 2.5K ohm at 20C.

If the Porsche part number is 930 606 013 02, the Bosch part number should be 0 280 130 059.

How is your fuel filter?

[Tall02]

Is the engine bucking only when under load? Does it idle smooth? If you increase the engine rpm when setting still does it buck?

One thing I would check is the fuel pump. Maybe when it is cold your supply pressure may be low or irratic. Find some one that can tap into the fuel bar with a pressure gauge and evaluate the pressure when it is cold compared to when it is warm.

Good luck

[Traveller]

Originally Posted by Tall02

Find some one that can tap into the fuel bar with a pressure gauge.

Harbor Freight has a fuel pressure gauge set with fittings that goes on sale for under $20. I don't know if any of the supplied fittings would work, but one of them can always be brazed to the fuel rail pressure test point cap that one should always have as a spare.

[America Roadste]

The fuel pump is new. Will ask the technician to check the fuel pressure and will ask when he last replaced the fuel filter.

[Traveller]

Originally Posted by America Roadste

The fuel pump is new. Will ask the technician to check the fuel pressure and will ask when he last replaced the fuel filter.

They last for many miles/years. Of course it'll always be dependent on the quality/cleanliness of gas. I only suggested that considering your mileage and the possibility that it may never have been changed. The fuel filter that I changed last winter had a date code 30.03.95 making it the original fuel filter that came with my 06/95 993. Mind you, it only had about 42K miles on it.

The pressure test will also confirm the pressure regulator.

[America Roadste]

The bucking happens under load or at idle. It only happens within the first 10-15 minutes after starting for the first time of the day. After two or so turn-off the ignition and restarting, it runs great and of course, by then the car is much warmer. The Porsche technician unplugged the cylinder head temperature sensor and cleaned the contacts yesterday evening. I have not started the car today for driving but will do so tomorrow. I am curious and hopeful that will cure the problem.
Thanks to all for the help and will get back tomorrow with an update.

[Tall02]

The only other item that could cause trouble when cold is your idle air valve . If it is sticking during the cold stage it may not be allowing enough air to the engine. You can take this off the intake manifold and clean the inside movable valve with a carb cleaner.

Good luck

[America Roadste]

Well, cleaning the contact on the cylinder head temperature sensor did not help. But yesterday, after first start in the morning, the car was idling while I was doing something else and twice the revs increased for a short moment and then returned to idle RPM. The second one I watched and the RPM went from around 700 to 1700-1800. I then drove the car to the technician who cleaned the contacts for the electrical connection for the Mass Air Flow Sensor.
I forgot to mention the idle air valve, but will when I next see him.
This is surely a slow, painful process, but as I mentioned, I am no longer interested in changing out parts hoping that we hit on the culprit.
I do appreciate all help as I have already saved money by using your advice.

[Traveller]

Originally Posted by America Roadste

I am no longer interested in changing out parts hoping that we hit on the culprit.

Then you will have to test the parts that are suspect.

You might have electrical contact issues. Have you removed and re-installed all connectors? Have you visually inspected all pins & sockets within each connector under good light?

You might try using Würth Contact Cleaner.

[America Roadste]

Agree with testing culprits. The technician is using a contact cleaner as he reviews suspect items.

[surfdog4]

i haven't pulled the 964 Jetronic diagrams out, but the aux air valve sticking could definitely cause problems like you describe (I had an '87 944 that had those same symptoms). If i remember correctly, there is also another fuel injector that provides more fuel during warm up that is also thermally activated (check me though, my experience there was on a 928, not sure this holds on the 964)

[viking66]

I had something similar on my previous 964 which took me a while to figure out. The cause was that the throttle body return spring had gone weak over the years resulting in that the micro swith for the idle valve activating intermittently and sticking.

Shortened the spring slightly and replaced the micro switch and lived happily forever after.

Good luck

[America Roadste]

Thanks for the tips and will have the tech check.

LB

[America Roadste]

All,
it turns out the Porsche technician's diagnostic machine was faulty and once fixed it showed a faulty cylinder head temperature sensor. He replaced the sensor and I am back in business with a perfect running car. Thanks to all for your assistance with this problem. A special thanks to Traveller.