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| 83 944 cutting out with throttle http://forums.tipec.net/viewtopic.php?f=25&t=23688 |
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| Author: | RWBennett [ Mon Nov 19, 2018 5:45 am ] |
| Post subject: | 83 944 cutting out with throttle |
Hello Everyone, I wanted to bow my head and humbly ask for help with my otherwise great 83 Black on Black 944. The car is a great survivor and has me completely stumped. I bought the car as non running project car and started to sort it out. The car had a few problems initially one of which was that it would cut out with application of throttle, if you cracked the throttle, it would rev up to a point cut out, die down to a lower RPM, cut back in and rev-up, then cutout again and repeat...sort of a rev....pause.....rev....pause etc. cutting out about 2-3K and cutting back in about 1500RPM. I knew I had to fix this but I put this problem to the side as the bigger problem was a fried clutch and leaking head-gasket. I did an engine swap with a used-tested engine and replaced the clutch, did a water pump and belt service, so I know the timing is correct. I also setup the gaps on the speed and reference sensor while the bell housing was off. Got the engine in and still the hit and miss running. And when I say hit and miss, I mean like you cut the ignition off and on. Since that time I have checked and verified the following with no correction to the hit and miss. Fuel pump - checked OK - pressure at injector manifold 30psi idle and 40-45 with throttle. Fuel filter - replaced both external and in tank screen Fuel pressure regulator - was leaking a bit of fuel in the vacuum lines - replaced with a new one Fuel injectors - replaced with new Ignition - new plugs, wires, cap and rotor - ignition isn't cutting off during the hit and miss....verified with ignition light and has HOT spark. New Battery and cleaned ground cables. Replaced AFM with rebuilt one - no change in symptom or problem Swaped DME with tested rebuilt unit - no change in symptom or problem Changed DME harness with a different one, New speed and reference sensors I've checked the speed and reference sensors at the DME plug and get correct static Ohm readings and correct dynamic voltages I've tested with and without TPS connect (was a different one with the replacement engine) no change Tried unplugged TPS and no change in hit/miss (worse idle when unplugged). Changed the temp sensor - no change The car will start right up every-time, settle into a great idle and will idle all day long, slight throttle crack and it revs, big throttle moves and it hit/misses nowhere approaching any sort of rev-limit. Will all the new parts it is running great up to the cut-out. I'm posting this request on a few Porsche websites as I'm very desperate and becoming quite frustrated by this behavior. |
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| Author: | scm924s [ Wed Nov 21, 2018 11:23 pm ] |
| Post subject: | Re: 83 944 cutting out with throttle |
Sounds a bit like fuel starvation at high revs, check the condition of the fuel hose between tank and pump, may be collapsing internally, also check that the tank vents aren't blocked causing a tank vacuum, try running with cap loosened off. |
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| Author: | Prophead [ Thu Nov 22, 2018 9:34 pm ] |
| Post subject: | Re: 83 944 cutting out with throttle |
scm924s wrote: Sounds a bit like fuel starvation at high revs, check the condition of the fuel hose between tank and pump, may be collapsing internally, also check that the tank vents aren't blocked causing a tank vacuum, try running with cap loosened off. I'd second some kind of fuel leak or restriction. Could be a squashed pipe under the car. Have you checked / changed the Fuel rail Damper? It works in partnership with the FPR, might be doing something funny with the fuel rail pressure. Would be worth getting a fuel pressure guage on the rail to see what it reads when you get your issue. |
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| Author: | RWBennett [ Fri Nov 23, 2018 12:57 am ] |
| Post subject: | Re: 83 944 cutting out with throttle |
Hi..yes I checked the fuel lines no problems. I also tried with fuel cap off to ensure tank venting. I also installed a brand new Fuel pressure regulator and also installed a Lindsey Racking fuel pressure gage directly on the injector rail and verified sufficient fuel pressure, I'm maintaining 40-45 PSI during the cutting out. the pressure is stable so I know the damper is working. I've also tried a noid light on one of the injector plugs and can see the light pulsing bright/dim with the cutting out causing me to think the injectors stop firing. Spark has been verified super hot and keeps firing constant during the cutting out, I tested with an insight and timing light, both indicate it keeps firing when cutting out, no pulses. Today I tried a new battery to make sure it wasn't a current problem, no change. I then disconnected the alternator (car turned off) and cranked and ran on battery only, no change. I thought it might be a failing regulator putting out electrical noise messing with th DME, but it had no effect on the problem. I'm going to bypass the alarm tonight to ensure it's not flaking out or causing the issue. |
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| Author: | scm924s [ Fri Nov 23, 2018 11:58 am ] |
| Post subject: | Re: 83 944 cutting out with throttle |
You mention you have replaced DME with rebuilt unit. i am assuming that is the main Motronic unit, what about the DME relay? |
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| Author: | RWBennett [ Sat Nov 24, 2018 5:48 am ] |
| Post subject: | Re: 83 944 cutting out with throttle |
I pulled the DME Relay and desoldered all the connections for good measure and I don't think I'm loosing power during the cutting out. I'ver ordered a replacement one and will try the replacement when it arrives. |
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| Author: | RWBennett [ Tue Nov 27, 2018 1:03 am ] |
| Post subject: | Re: 83 944 cutting out with throttle |
Hi all, I wanted to share with you what was wrong and how it was corrected. When I first got this car the AFM had it's cover missing and the wiper arm was bent and crooked. So one of the first things I did when I got the replacement engine installed was to replace the AFM with a rebuilt unit from one of the online Porsche parts resellers, an AFM that was remanufactured by "Fuel Injection Corp". After chasing gremlins for a couple of months, I kept thinking what could be causing my injectors to shut down off idle......It haunted me. I chased wiring harness, DME, grounds, injectors, etc....nothing fixed it. Just for grins, I bought a second hand used AFM to try and it solved the problems I was having with the car. I put the used AFM on and instantly it was fixed. The rebuilt unit was the culprit, as it is either defective or the wrong unit for my application. Strange thing is the cover has the right Bosch part number but the "Fuel Injection Corp" part number isn't the one they show on their website. Their website lists their part number for my year/model as 412028 however as the picture below shows, this unit as the number 28921 on the label. So what's up? I don't know but I"m going to write them and let them know how this hosed me up for months, and give them my personal thanks. Thanks to everyone who took the time to help me and I appreciate it very much. |
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| Author: | RWBennett [ Tue Nov 27, 2018 1:11 am ] | ||
| Post subject: | Re: 83 944 cutting out with throttle | ||
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| Author: | scm924s [ Tue Nov 27, 2018 11:57 am ] |
| Post subject: | Re: 83 944 cutting out with throttle |
Glad it's sorted, very frustrating. |
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| Author: | scam75 [ Tue Nov 27, 2018 12:01 pm ] |
| Post subject: | Re: 83 944 cutting out with throttle |
Excellent news RWB! A lot to be said for parts "re-manufacturers". I've had 2 alternators from an alleged reputable "re-manufacturer" and the first one shot sky high voltage off the chart, in excess of 15/16v at times. Car never like that, so I thought I'd change the regulator as I had a spare, very long job on a turbo removing the alternator and the regulator can't be changed in situ. All back together, still same. So he sends me another which lasted about a month before total bearing failure. Never again! The best thing I ever done with my AFM was launch it over a hedge when I installed a MAF, although I wouldn't bother with that on an N/A. Glad you got it sorted! Stuart |
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