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?

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

MPS diaphragm cracked, check if it holds vacuum.

Check fuel pressure engine idling and revving. 29 psi or thereabouts is spec. Leaking MPS and defective CHT sender can cause rich running.

^^^that my forst guess too. And easy to check.

Doesn't explain the moaning though....did you hit a squirrel?

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?

Could be a failed fuel pressure regulator

Check the plugs first to see if it is cylinder specific. Check the Mps of course. Do you still have the decel valve?

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.

Djet, so no flap.

Do a valve adjustment

Might be both symptoms

Afm adjusted? Djet? Wait, what?

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.

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.

CHT checks out. MPS has no vacuum, so I guess it is deceased.

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.

@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

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.

@JeffBowlsby will rebuild your MPS for you also!

Thank you all.

http://www.914world.com/bbs2/index.php?showtopic=363220

@914sixer has one listed for sale

Sold and shipped today.