Fuel Problem?, Need Diagnostic Help |
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Fuel Problem?, Need Diagnostic Help |
Jim C |
Jan 15 2023, 12:23 PM
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#1
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Member Group: Members Posts: 190 Joined: 11-July 19 From: Texas Member No.: 23,294 Region Association: Southwest Region |
I left on a 200 + mile drive yesterday morning with other aircooled Porsches. As usual the D jet equipped '76 2.0 was running beautifully. Until I got within 10 miles of home. At that point the engine tone changed and I experienced small backfires occasionally. At idle I could also hear a low moaning sound inside the car on the left side. Besides running rough, although with good acceleration, there was black smoke and lots of carbon in the tailpipe and the smell of fuel in the exhaust. I thought exhaust leak maybe, but when I put my hand over the tailpipe the engine stopped. The car has had extensive work: FIs cleaned and checked in 2019, rebuilt MPS 2020, ECU tested and functioning, new CHT 2022, Bowlsby engine harness 2022, 123 distributor 2021, all vacuum lines replaced, AFM adjusted 2022. She will start as normal and run, although roughly.
Fuel pump perhaps? |
brant |
Jan 15 2023, 12:40 PM
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#2
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914 Wizard Group: Members Posts: 11,623 Joined: 30-December 02 From: Colorado Member No.: 47 Region Association: Rocky Mountains |
Black smoke and the smell of fuel would make me think rich (fuel injection)
Or ignition Check all of the plug wires with a timing light Check that the timing hasn’t changed and the distributor clamp is still tight |
mgphoto |
Jan 15 2023, 01:02 PM
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#3
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"If there is a mistake it will find me" Group: Members Posts: 1,339 Joined: 1-April 09 From: Los Angeles, CA Member No.: 10,225 Region Association: Southern California |
MPS diaphragm cracked, check if it holds vacuum.
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Lockwodo |
Jan 15 2023, 02:50 PM
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#4
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Member Group: Members Posts: 200 Joined: 23-December 21 From: Santa Cruz, Californnia Member No.: 26,193 Region Association: Northern California |
I left on a 200 + mile drive yesterday morning with other aircooled Porsches. As usual the D jet equipped '76 2.0 was running beautifully. Until I got within 10 miles of home. At that point the engine tone changed and I experienced small backfires occasionally. At idle I could also hear a low moaning sound inside the car on the left side. Besides running rough, although with good acceleration, there was black smoke and lots of carbon in the tailpipe and the smell of fuel in the exhaust. I thought exhaust leak maybe, but when I put my hand over the tailpipe the engine stopped. The car has had extensive work: FIs cleaned and checked in 2019, rebuilt MPS 2020, ECU tested and functioning, new CHT 2022, Bowlsby engine harness 2022, 123 distributor 2021, all vacuum lines replaced, AFM adjusted 2022. She will start as normal and run, although roughly. Fuel pump perhaps? Check fuel pressure engine idling and revving. 29 psi or thereabouts is spec. Leaking MPS and defective CHT sender can cause rich running. |
JeffBowlsby |
Jan 15 2023, 03:21 PM
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#5
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914 Wiring Harnesses Group: Members Posts: 8,503 Joined: 7-January 03 From: San Ramon CA Member No.: 104 Region Association: None |
MPS diaphragm cracked, check if it holds vacuum. ^^^that my forst guess too. And easy to check. Doesn't explain the moaning though....did you hit a squirrel? (IMG:style_emoticons/default/wink.gif) |
Jim C |
Jan 15 2023, 05:39 PM
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#6
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Member Group: Members Posts: 190 Joined: 11-July 19 From: Texas Member No.: 23,294 Region Association: Southwest Region |
No animals were harmed or ingested. Could the noise be a failing fuel pump? All the other items mentioned in the replies so far were replaced since 2019. How long is this stuff supposed to last before it fails?
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ChrisFoley |
Jan 15 2023, 07:01 PM
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#7
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I am Tangerine Racing Group: Members Posts: 7,925 Joined: 29-January 03 From: Bolton, CT Member No.: 209 Region Association: None |
Could be a failed fuel pressure regulator
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emerygt350 |
Jan 15 2023, 07:49 PM
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#8
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Advanced Member Group: Members Posts: 2,091 Joined: 20-July 21 From: Upstate, NY Member No.: 25,740 Region Association: North East States |
Check the plugs first to see if it is cylinder specific. Check the Mps of course. Do you still have the decel valve?
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Rikyrat |
Jan 16 2023, 11:15 AM
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#9
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Member Group: Members Posts: 58 Joined: 30-January 19 From: Phoenix Member No.: 22,844 Region Association: None |
On my first 914, I am on my third, I had a similar problem, when I took it to the mechanic, he found that circuit board on the air flow sensor was worn, we replaced it and the problem was solved.
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emerygt350 |
Jan 16 2023, 11:32 AM
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#10
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Advanced Member Group: Members Posts: 2,091 Joined: 20-July 21 From: Upstate, NY Member No.: 25,740 Region Association: North East States |
Djet, so no flap.
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brant |
Jan 16 2023, 02:02 PM
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#11
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914 Wizard Group: Members Posts: 11,623 Joined: 30-December 02 From: Colorado Member No.: 47 Region Association: Rocky Mountains |
Do a valve adjustment
Might be both symptoms |
emerygt350 |
Jan 16 2023, 08:13 PM
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#12
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Advanced Member Group: Members Posts: 2,091 Joined: 20-July 21 From: Upstate, NY Member No.: 25,740 Region Association: North East States |
Afm adjusted? Djet? Wait, what?
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Jim C |
Jan 17 2023, 07:57 AM
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#13
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Member Group: Members Posts: 190 Joined: 11-July 19 From: Texas Member No.: 23,294 Region Association: Southwest Region |
Still have decel valve installed. I have a vacuum tester on the way and will test the expensive, less than two-year-old rebuilt MPS. Will also check plugs and fuel pressure (I assume there is a thread here somewhere on how to pressure test). Will go from there. I can spend a lot and get this running beautifully for 6 months to 8 months and then something seems to come out of nowhere. Frustrating.
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Superhawk996 |
Jan 17 2023, 08:14 AM
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#14
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914 Guru Group: Members Posts: 5,812 Joined: 25-August 18 From: Woods of N. Idaho Member No.: 22,428 Region Association: Galt's Gulch |
Just for fun, go check the CHT sensor - aftermarket parts have seen early failures by other members.
If the CHT goes bad to the high end resistance or becomes disconnected electrically at the spade connector (open circuit = infinite resistance) or within the wiring it will think engine is cold and adds fuel and would cause a rich condition. |
Jim C |
Jan 17 2023, 08:22 AM
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#15
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Member Group: Members Posts: 190 Joined: 11-July 19 From: Texas Member No.: 23,294 Region Association: Southwest Region |
Just for fun, go check the CHT sensor - aftermarket parts have seen early failures by other members. If the CHT goes bad to the high end resistance or becomes disconnected electrically at the spade connector (open circuit = infinite resistance) or within the wiring it will think engine is cold and adds fuel and would cause a rich condition. Will do. |
Jim C |
Jan 17 2023, 04:25 PM
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#16
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Member Group: Members Posts: 190 Joined: 11-July 19 From: Texas Member No.: 23,294 Region Association: Southwest Region |
CHT checks out. MPS has no vacuum, so I guess it is deceased.
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brant |
Jan 17 2023, 04:31 PM
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#17
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914 Wizard Group: Members Posts: 11,623 Joined: 30-December 02 From: Colorado Member No.: 47 Region Association: Rocky Mountains |
so your running extremely rich
most people can not even get their car to start with a blown MPS you are luckier than most.... rebuilding also requires recalibration. the absolutely best way to do that is not to set to the factory settings for gasoline that no longer exits... but to use a wideband AFR to tune it to your actual motor. takes a while... lots of hours and lots of work but your choices are use a rebuilder/buy used or buy a new diaphragm, install it yourself, buy an AFR gauge and calibrate it yourself. |
Dave_Darling |
Jan 17 2023, 04:53 PM
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#18
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914 Idiot Group: Members Posts: 14,985 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California |
@JeffBowlsby , above, sometimes will calibrate a batch of MPSes. Or at least, he has in the past. I don't know if that's still the case, but he can comment.
I think Tangerine Racing will rebuild it for you. They also will sell you a new diaphragm and instructions on how to rebuild it yourself, if you prefer. --DD |
GregAmy |
Jan 17 2023, 04:54 PM
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#19
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Advanced Member Group: Members Posts: 2,296 Joined: 22-February 13 From: Middletown CT Member No.: 15,565 Region Association: North East States |
CHT checks out. MPS has no vacuum, so I guess it is deceased. https://tangerineracing.com/shop/ols/produc...t-mps-diaphragm Let Chris rebuild it. Totally transformed the driving experience, so it was that way for a while. |
bzettner |
Jan 17 2023, 09:46 PM
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#20
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Member Group: Members Posts: 211 Joined: 2-June 13 From: San Antonio, TX Member No.: 15,951 Region Association: Southwest Region |
@JeffBowlsby will rebuild your MPS for you also! (IMG:style_emoticons/default/aktion035.gif)
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