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Car Forum / Porsche / Porshe 944 / September 2006944 no start when hot
I have a 8 month old FPR with correct BOSH-#, could the fuel pump cause the fuel pressure to still be too high, since it's not an original part as far as I figured out? This is the case: When engine is cold I have no problem starting the car. This is an early 1985, i.e. 8V N/A (old dash). At idle the fuel pressure (permanently measured at end of fuel rail) fluctuates between 38 and 40 psi. Seems high? Should be around 29 psi. When turning the engine off, the fuel pressure immediately rises to ~46 psi. This is also the pressure when the car does not start. The only way I can start the car and make it run (very bad) is by removing the fuel pump fuse. The pressure lowers to 20 psi and it runs (almost), sometimes for up to 30 seconds. Connecting the fuse while engine is running in this 'mode', will cause the engine to shut down immediately and pressure to rise again to 46 psi! I have a feeling that a warm DME causes the injector drivers to immediately shut down when pressure rises above ~38 psi. This is not the case during a cold start? Now, I am aware of that the normal pressure for early 1985 should be around 29 psi. But I replaced the FPR 8 months ago, and the car has started normally up till the last month. Now it always hesitates when warm. I suspected my pressure gauge was faulty and offset by ~10 psi? This is probably not the case? I looked at my fuel pump. It's green (not black), and the Bosh # is not 0580 464 069 as I think it should be? Can someone verify that this is the correct part # I should have? My number is 0580 464 028. I think the previous owner has changed pump, (the injectors were also not original) and now the pump is stronger. Yesterday I managed to start the car (when warm) by clamping the fuel line prior to the dampener and thereby lower the pressure to around 36 psi when engine was running at idle! Now, again my question, even if I have a new FPR with correct BOSH #, could the fuel pump cause the fuel pressure to still be too high? I thought the FPR 'removed' the over-pressure condition? Furthermore I 've checked my vacuum lines, and they seem OK (fuel pressure rises when removing the vacuum line etc). The second method of starting the car (when warm) is to tow it, and start it on second gear. This may be because a hight vacuum is created which causes the FPR to create less pressure on fuel rail? Tnx / Anders There is a check valve on top of the fuel pump that keeps the pressure up when you stop the engine. If it does not work then the pressure falls and engine will not start immediately. Also, sounds like the FPR is not working. Check the vacuum line rubber Y and other connections to make certain FPR is receiveing vacuum. > There is a check valve on top of the fuel pump that keeps the pressure > up when you stop the engine. If it does not work then the pressure > falls and engine will not start immediately. > Also, sounds like the FPR is not working. Check the vacuum line rubber > Y and other connections to make certain FPR is receiveing vacuum. Tnx a lot for quick response! I changed for another old but known ok FPR, and the pressures were the same. I think my gauge is offset by 10 psi somehow. I checked the vacuum lines. They seem ok, and when I pull the one on the FPR the pressure increases about 6-8 psi. You can hear the vacuume surge through the small line. However this evening I found another way of stalling the engine. I slowly pinched the fuel return line, the pressure increased slowly to over 60 psi, and then the engine stopped? DME injector driver shut down? However, not the engine was impossible to start again, except by removing the fuel pump fuse, and thereby running the engine with very low fuel pressure for a while? I am confused now. What's going on. DME problem? You said it ran fine for eight months after changing the FPR. My guess is you have a blockage in the return fuel line. I believe the DME will shut the engine down if pressure goes too high. Maybe the dampner is causing the problem. Try here for more informtion: http//www.frwilk.com/944dme/ Tnx a lot for the input darthpup! I will disconnect the return line and let the fuel flow into and external container and check if the pressure lowers on the fuel rail, and if there seem to come sufficient fuel into my container. You are right about the DME can cause the shut down. However, I read a topic here where someone had problem with the DME giving a pulse signal, but not sufficient for the injectors to spray, (he put diodes on the signal cable on the fuel-rail). I have not verified that this is the case. So I am also suspicious about the DME still. Regarding the dampner. There is so little written about it. My pressure, fluctuates about 2 psi when the car is running in idle. I am not sure if this is normal. To me, it does not sound too much? / Ajan darthpup skrev: > You said it ran fine for eight months after changing the FPR. My guess > is you have a blockage in the return fuel line. I believe the DME will > shut the engine down if pressure goes too high. Maybe the dampner is > causing the problem. > > Try here for more informtion: http//www.frwilk.com/944dme/ Dampener? Try removing the vacuum line to it and check. If you have some bastard set of injectors that could be your problem. Previous owner could have taken a set from another engine, became discouraged with performance and sold you the car? However, as you pointed out, the engine ran fine for eight months so you have developed the problem recently. You need to get the fuel pressure down in any case. I have installed new pumps which have shown up to 100 psi before activating the FPR. Tnx for input darthpup! > Dampener? Try removing the vacuum line to it and check. > If you have some bastard set of injectors that could be your problem. [quoted text clipped - 5 lines] > I have installed new pumps which have shown up to 100 psi before > activating the FPR. Fuel pressure is the same with the fuel return line disconnected. Also, when the engine runs and I disconnect the vacuum line, the pressur rises about 8 psi. But the engine does not die? Maybe the DME is OK after all. I run into the non-start situation once more, i.e. the car starts and runs with fuel pump disconnected. While inspecting the vacuum lines ones more, I pressed the brake pedal a couple of times. There was a 'woosshing' sound the first steps, and then it dissapeared. After doing this, the car started immediately and run with no remarks? Can my brake booster cause my problem, i.e. air is coming in via the brake booster, causing the fuel/air mixture to be to lean. The brake booster hoose, connects via y-connector to the intake manifold and the big hoose between the AFM and the throotle housing. Can the brake booster cause such intermitent problems? Furthermore, I measured my vacuum at the small vacuum line at the Fuel pressure regulator. It was high and steady at 22 mmHg. This is OK I think? Please any input is valued enourmeously. Note also that the brake booster appears to be working normal, cause if pressing the pedal several times prior to ignition on, and holding the brake pedal while starting the engine, causes the brake pedal to travel a little bit down, which I think is normal? However, this procedure helpt a non-start situation to be a perfect start?? I am even more confused. Regards / Ajan Whoosing from brake boost is not correct. Try removing the hose from brake boost, plugging the hose end and see if your problem disappears. Also, your vacuum gauge on dash, MPG meter, should be reading 30 at idle. My guess from what you have described is an intermittant loss of vacuum which would cause the fuel pressure to go too high from time to time, without prediction. The diaphraghm in brake boost may be faulty. You must have correct vacuum to FPR to solve your problem. Tnx a lot for the input darthpup Plugging the brake hoose is definitely the next thing I will try. I guess I have to be careful driving since (quite) a bit more brake power is needed? Now, 30 mpg, I can't confirm I have that. I think mine says 4 liter/km but I think I have both scales on the meeter. I will check that tomorrow. It is on the second vertical line on that backward scale. If I have an intermittant vacuum leak in the brake booster, couldn't it also cause the air/fuel mixture go very lean all of a sudden, causing the engine to stop, or the no-start condition when engine is cold, AAV open (Auxiliary air valve) and brake booster is not behaving. Just a thought. Again, tnx a lot. You keep me busy trying new ideas. Great! / Ajan Just an after thought. You could run a separate vacuum line directly from the vacuum fitting on intake to the FPR . In any event you have a vacuum leak somewhere. There is a Y fitting under the air cleaner which typically breaks with age from heat. Hard to get to but could be problem. You need to find the vacuum leak and bring fuel pressure down to 29-35 psi. Do not confuse test with repair. Driving with impaired brakes may not be wise. Which y-connector do you refer to. Under the air-cleaner? There is one y-fitting splitting the small vacuum lines going to FPR and FD. But I guess you mean the venturi tube, item 18 in the link below which goes to Brake booster and inlet manifold and comes from the big hoose between the AFM and TB via item 21 in the link below? Am I right? I think that y-connector is under the intake manifold (more or less). Well maybe it is close to the aif filter system (prior to AFM). http://www.pelicanparts.com/944/944_parts/944_83-85/Pic70.jpg Also I am having problem following where hoose 13 and 15 ends up in my car. It is extremely difficult to figure out where the hooses go underneath the intake manifold. Please hava a look at the link: http://www.pelicanparts.com/944/944_parts/944_83-85/pic7.jpg Hose #12 comes from the big hose between AFM and TB I beleive and then goes to the AAV. But after the AAV, it looks like hose #15 connect to the inlet manifold which makes sense (extra air during cold start), but why is the hose split. Where does the other end go? Also hose # 29 I don't recognize and the item 16 electric air valve? What does that do? I have too look underneath the intake manifold again this evening. I seem to miss some components? Sure, I will test it very carefully. Check the diagram at http://www.clarks-garage.com and you will see the other y connector. However, from what you have described I believe your problem is the brake booster. Tnx for the link darthpup. It seems like the diagram is for a 1985.5 and mine is an early, but it was very helpful. I checked the vacuum hooses again last night, and I also plugged the vacuum line from the brake booster. No difference, and I could not find any leak either? However, I noted that my economy gauge starts to show very low fuel consumption. Mine has a different scaling than yours so I can't really compare. I thought however that the gauge was connected to the speedometer (on transaxle) and DME/Tach and had nothing to do with the vacuum? However I checked the speed and ref-sensor on the back of the engine. The speed sensor had a sine-wave with 10 Volt peak to peak. Looking in Clark's garage it should be a saw-tand pulse? Is the shape of the pulse important? The ref-signal is also strong. Is there anything that can be loose on the flywheel or could my sensors still be missadjusted? It does not make sense to me however. The car usually start when disconnecting the fuel pump fuse? I am so confused now. Maybe I should try to take it to a shop? Your problem is quite simple. It is poor vacuum to FPR. You need to find the leak and bring the fuel pressure down. darthpup skrev: > Your problem is quite simple. It is poor vacuum to FPR. You need to > find the leak and bring the fuel pressure down. I'm not so sure about the vacuum problem darthpup. The problem has escalated. Yesterday I started the car normally, it died after 4 minutes. Restarted, it run for 1 minute and then the economy meter bounced up to 'lots of fuel consumed' on idle???? I re-checked the DME connector pin #13. Infinite resistance? It should be around 2 kohm at room temp? I disconnected the DME temp sensor from the cable harness. Again infinite resistance. Also put my ohm meter between the cable connector at sensor end and at the DME end. The resistance varied betwen 30 ohm and 150 ohm when I moved the connector. With those results the DME probably thinks the engine is dead coold, and this explains why my economy meter all of sudden jumps up and the engine dies. I obviously have some serious trouble. I hope it is the DME connector itself. I found a screw at the left most end of it. I haven't opened it yet. Anyone having experience in replacing the connector or the whole Motronic cable harness? Thanks for reading I have not had any problems with my DME but there are numerous posts here with owners finding cold or poor solder joints in the DME. You might try heating up some ot the joints to resolder them inside the DME as others have done. Even better would be to try and locate a working DME and substitute for test. Rebuilt units are expensive. Also, test the thermal sensors in the system. One could be shorted out. Yes, my DME temp sensor seems to have infinite resistance towards ground. I've disconnected the DME connector when doing so. Currently I'm not suspicious about my DME, but the male connector attaching to the DME, and of course the temp-sensor itself. Tnx darthpup. darthpup skrev: > I have not had any problems with my DME but there are numerous posts > here with owners finding cold or poor solder joints in the DME. You [quoted text clipped - 3 lines] > Also, test the thermal sensors in the system. One could be shorted > out. start by checking the temp sensor - the connectors to the sensor frequently corrode - though that wouldn't explain why resistance varies as you wiggle the wires at the connector, I'd still start by testing the sensor and checking the connections at the sensor end > Yes, my DME temp sensor seems to have infinite resistance towards > ground. I've disconnected the DME connector when doing so. Currently [quoted text clipped - 12 lines] >> Also, test the thermal sensors in the system. One could be shorted >> out.  SignaturePosted via a free Usenet account from http://www.teranews.com Tnx for input! I cleaned all connectors at both ends and the DME temp sensor itself. Got stable readings. The car starts and idles normally now. I will go for a testdrive tomorrow. I think this was the source of my problem, i.e. no fuel pressure related problem at all. I guess the engine died due to too much fuel injected, because the DME thought it was dead cold, due to the bad sensor readings. The car would just not start when engine was hot and the sensor said -300 degrees :-) and the injector were open way too long. It was however running with fuel pump fuse removed. Hope this is end of story, tnx for help and ideas during my struggle Regards / Ajan |
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