944 Starting problems

[drparr]

Having 944 starting problems, but only when engine is warmed up. Here is an email I sent to Vertex parts but did not get a reply to. Please advise which part (s) I need to fix this annoying problem. Many thanks, drparr.


Re: 1984 Porsche 944

Engine WP022294ZEN465584
Body WP0AA0945EN465584




Attention: Vertex 944 Specialist / Engine Sensor Specialist


Dear Vertex,



Received the new radiator you expressed down to me and installed it. New radiator is working great- cooling engine just fine. Engine temp gauge on dash in normal operating range.



Now, here is the problem. On the weekend before you shipped my new radiator, my 944 engine overheated for about 5 to 10 minutes but I did not notice that till I got home. This was due to the old radiator having a bad leak and most of my coolant leaked out while driving. I was driving for maybe 5 to 10 minutes with an engine temp. gauge on the dash showing overheating (into the overheating zone) before I got home and noticed the leak. I estimate this amount of time because I started the car cold and drove for about 20 minutes (normally takes about 10 minutes for engine to heat up to where engine temp gauge goes to right past the middle).



So the situation is that the engine ran really hot for about 5 to 10 minutes.





1) I checked the oil- it was down a quart and I topped it up. Now I check the oil level periodically and it is perfect- it is not burning or losing any oil.

2) The car starts beautifully in the morning when I go to work. Engine compression is good, oil pressure is good and normal on gauge as it is running.

3) The car starts beautifully after I let it sit for about 2 hours and it cools down completely and I need to drive it again.

4) The car starts ok if I do a short 5 minute hop to the grocery store, park it for a minute and then start it up to go back home. (the dash engine temp gauge can be in the middle or almost middle position).

5) The new radiator is cooling the car great. The engine runs beautiful and smooth. I can drive the car for an unlimited time and the cooling and engine is running great. The dash gauge showing engine temp stays in the normal operating range the entire time, a little to the right just past middle. The cooling fans cycle on and off as required normally.

6) The starter and alternator and battery is great. They crank the engine perfectly and the battery is like new and the alternator is charging perfectly.

7) The spark plugs are great- they are new Bosch Platinums installed 3 months ago. The car runs great, accelerating smoothly and powerfully- everything normal in the ride.

8) HOWEVER, if I run the car for say 15 minutes or longer (the dash gauge showing engine temp is in the normal operating range, tilted towards the right just past middle), park it, shut it down and THEN try to get in and start it again to go somewhere else, THE CAR ENGINE WILL NOT START. The starter turns the engine great, the battery is great but some sensor is not allowing air or gas or some combination to get to the engine to fire. The engine cranks and turns beautifully but it will not fire. I can crank that engine repeatedly for 30 seconds at a time and it turns beautifully but will not start. (I can press the gas pedal down while cranking and same thing- will not start). I literally have to wait about 1 hour to let the engine cool down (the dash gauge shows the engine temp in the far left area warm-up range) and then it will start again beautifully. (I do not smell any gas leak or anything when I am cranking the hot engine- it turns over great and sparks but just will not start).



So a little elimination first before we decide what it could be:



A) Battery, Alternator, Starter, Spark Plugs, Engine Compression, Thermostat: No, it is not the battery, not the alternator, not the starter, not the ignition, not the spark plugs, not the engine compression. It is also not the thermostat because the engine temp gauge is in normal range after long driving and I checked under the hood- the overflow tank is taking up the expansion (I can see more coolant in the overflow tank after a long drive as is expected. The level is higher (max. mark) than when I check it before going for a drive (min. mark) ). So that means the thermostat is opening and coolant is circulating through the engine.



B) Distributor cap, Rotor, Ignition wire: Can’t be as the car does start when it is COLD and it runs smooth and does not misfire when it is just started or when it has been running hot for a long time.



C) Fuel pump or fuel pump fuse: No because the car does start when it is COLD and it does run once it is started.



D) Fuel Injector: I don’t think it is the fuel injector as again the car does start when cold and the engine does run well, not poorly- it has great acceleration and instant pick-up- very responsive.



E) Idle speed regulator: It can’t be that because the car starts and idles beautifully, cold or hot.



F) Ignition DME Unit: can’t be that because the car sparks and starts and it never misfires.



G) Ignition Coil: not that because the spark is good. The car starts great when cold and runs great.



So now we are left with the following possibilities and I need your help to decide which one it is:



1) Auxiliary Air Valve- would this cause erratic fuel injection ONLY WHEN HOT and cause the 944 to not start when HOT? (It starts great when cold and runs great though).

2) Fuel Pressure Dampner- could this be the part preventing the 944 from starting when HOT?

3) Fuel Pressure Regulator- Could this be the part not allowing the 944 to start when HOT?

4) Air Flow Meter- Could this be the part not sending enough air to start the car when it is HOT?



So which one could it be? Which one could have been damaged by overheating and now is malfunctioning? Remember that everything worked beautifully BEFORE this overheating incident I had so it has to be related to that overheating incident.



Appreciate your time and any advice you can email me back with so that I can order the faulty part from you and replace it. Anxious to hear from you about this problem as it is a drag not to be able to stop and do errands with the car- as it is I can only drive to work in morning and then drive home at night. Hoping you had another client with the same bad experience of his 944 overheating and damaging some part so he cannot start it when HOT and you can share the experience with me.



Best Regards,

[Lawrence Coppari]

I have a 951 that would not start occasionally. It always started when cold. Here is something you could check.

There are two magnetic sensors mounted at the top of your clutch housing. They feed vital information to the DME. About a foot from where the sensors are mounted is an electrical union. It is where the sensor leads plug into the cable that carries the crankshaft position/speed information. These plug in connectors get corroded. Open them, clean, and coat with a diaelectric grease. When I did that to mine, the problem went away and never came back.

This might be your problem. No guarantees, but it is worth a shot.

[Magnus_944]

Some wild guesses.
Check the DME relay, it can cause many strange problems. The check valve in the fuel pump can also be bad and cause hot start problems.

//Magnus

[cheap944]

My problem is absolutely identical to yours, except my car never overheated as far as i know. I have cheacked my fuel pressure and it is fine, so on my car i have ruled out the FPR. I am cluelless as to where to start, im about to replace my fuel pump for the check valve in it. If you get your problem fixed Please email me ill keep you posted on my car.

[AFJuvat]

Have you properly bled your cooling system? Car should be running in the lower 1/3 of the temp gauge under normal driving conditions. SHould only go above the half way mark in stop and go traffic.

Sounds like your DME relay and/or your aux air valve is shot

AFJuvat

[944TurboRacer]

I had a "warm" no start problem with my 944 turbo. The car would always start cold. But if I drove a short distance and shut the car off, there was a good chance it wouldn't start again. If I let it cool completely, the car would start up fine.

I was able to confirm that I had no spark when the car wouldn't start and that the fuel system was working fine.

Ultimately I discovered that the DME unit was the culprit. I found three broken solder joints on one of the DME power transistors. (There are three of these power transistors mounted to heat sinks along the inside wall of the DME unit). Once I resoldered these joints I have never had a "warm" start problem again.

Do an internet search of this problem. There is information out there and photos to assist in this repair.

[turbohead7]

If you want to fix your car you need to diagnose it properly. Get the car to mess up and then check for spark at the plug wires. If the engine cranks and you have spark, check to see if the injectors are clicking while someone cranks the engine over. They can be felt/heard with a long screwdriver/stethoscope. If you're missing spark or injector signal while cranking; the DME, DME relay or bellhousing sensors are all possible culprits. If you have injector signal, good spark, and adequate compression that engine has to fire----provided the proper amount of fuel is being delivered. Check cranking fuel pressure, running fuel pressure and residual fuel pressure. The injector signal itself is pulse-width modulated, otherwise known as "on-time". This time, measured in mS, depends on the Vane Airflow Sensor signal, coolant temperature sensor and O2 sensor. Although not likely the CTS could've been damaged by an overheat. First things first---see what you're missing and get back to us.

[todwic]

Are you absolutely sure you have compression? Or do you assume because it starts right off that compression is good? Unplug your 0₂ sensor and see if your engine changes drastically. Unplug your injectors 1 at a time and see if one cyl. doesn't change less than the others. If you have done a comp. check, what are the #'s? Do they change when its warm? Take your aux air valve off and toss it in the freezer. Does it close most of the way when cold cold (couple hours in freezer)? Change out you NPT II sensor. They're cheap and could it hurt to have a spare? I'm glad I did. Do you have a multi meter with RPM capability? Carry it with and when it doesn't start, hook it up and see if you have secondary ign pulse.
IT IS BAD TO RUN YOUR ENGINE IN THE HOT ZONE FOR EVEN JUST A LITTLE WHILE. I'll say it again because it bears repeating. These aluminum heads and blocks WILL NOT take operation in the yellow area of temp gauge. My opinion is that you either shot your head gasket or warped your head.

[zaxx81]

I just fixed a similar problem on my '83 944. It was having problems intermittenly starting. As many of the other suggested you might check your DME Relay (a.k.a. Fuel Pump Relay). This unit has two relays inside of it. One relay closing when the key is turned to start the engine. This in response closes the other relay which will be closed continuous, while the engine is on. Relays or any electro-mechanical device can be sensitive to temperature. The other relay being on continuous while driving can produce heat in the unit making relays stick. You could experience the same thing with a cold engine, but the weather be nice and hot. Hope it helps some. I got a replacement from a Porsche Dealer for $16. O'reillys and Autozone won't carry them.

[carsontc]

doing a search on drparr it looks like traded his 944 for a 99 boxter last year...but lotsa of great stuff came from this old tread...it's a keeper !

[todwic]

lol I didn't realize this post was dated.