Misfire under acceleration - understanding fuel pressures

[ChuckleBeast]

Hi everyone,

I have a new to me '78 RoW 911SC. And looking for help on fixing a misfire issue at ~3600 RPM's.

When I received the car it started right up and seemed to run great but almost within days of driving it I was getting a vibration/shake in the rear of the car that felt like I wasn't running on all cylinders.

1. Pulled the wires and tested them, one had gone bad, cylinder 5 wasn't firing.
2. Replaced the wires
3. Replaced the plugs as it had been ~8 years according to the paperwork I had

Car fired up just fine, vibration and shake issue had gone away. The car cold/warm starts without issue.

As I let the car warm up and when I push the car above ~3600 RPM's I'm still getting a hesitation/misfire.
I performed a smoke test, could not find any vacuum leaks and RPM's drop significantly when the oil cap is removed.
I completed a fuel pressure test this morning using an S&G Tester. Confirmed this is a Bosch 045 WUR.
results:

1. Temperature 23C
2. System Pressure 5 bar - Normal
3. Cold Control - 3.4 bar - High
4. Warm Control - 3.6 bar - Normal (Or is it?)
5. Residual - 1.9 bar after 30 minutes

Am I correct in thinking there is an issue with the WUR leading to the high cold pressure? What else should I be examining or looking at? Could the WUR be leading to the symptoms I am describing?

Thank you!

[ChuckleBeast]

bump for any advice

[theiceman]

have you considered it could be your CDI unit failing under load ?

[ChuckleBeast]

Originally Posted by theiceman

have you considered it could be your CDI unit failing under load ?

I thought about this, it looks like it's still the original Bosch CDI and coil pack. I've thought about swapping out to MSD Streetfire and pack while I've already done this work. The CDI still gives of a squeal, is there a way to test the unit under load conditions?

[boyt911sc]

Originally Posted by ChuckleBeast

1. Temperature 23C
2. System Pressure 5 bar - Normal
3. Cold Control - 3.4 bar - High
4. Warm Control - 3.6 bar - Normal (Or is it?)
5. Residual - 1.9 bar after 30 minutes

Am I correct in thinking there is an issue with the WUR leading to the high cold pressure? What else should I be examining or looking at? Could the WUR be leading to the symptoms I am describing?

Thank you!

Chuckle,

Yes. Your CCP (cold control pressure) is too high at 3.4 bar/49.0 psi. that is already WCP (warm control pressure). The spec for WCP is 49~55 psi. and you are missing the capability for enrichment. You have no RICH mixture capability with your out of spec WUR.

Tony

[ChuckleBeast]

Continuing to see an issue with what feels like a continuing engine misfire which is significantly worse under load around 3300RPM.

So I took apart my WUR and cleaned it. Looks like the mesh filter may have been blocked. My fuel pressures are now 100% to spec (Thank you Tony for all your help and guidance!)

Summary of work done so far
Fuel pressures now in range system, cold and hot
Validated no vacuum leaks with a smoke machine
Checked fuel injectors, all are running without issue
Replaced spark plugs
Replaced plug wires (Originally cylinder 5 was not firing, wire was toast, now working)
Checked mixture to 2.5% with a Gunson

Had a chance to take the car out this morning, and still running into this misfire issue. With the car running I was checking to make sure all connections were seated. As I moved the Green wire (that connects distributor to WUR, I think?) The car immediately died. With the car running again if I touch the connection I can hear the idle fluctuate. It also looks like someone had tried fixing it in the past. I am guessing this is a bad connection and the issue and is impacting the CDI? Am I in the ballpark?

Looking for validation that this could be an issue and how to try and fix this correctly. I have pictures below..


Tony[/QUOTE]





[QUOTE=boyt911sc;12090080]Chuckle,

Yes. Your CCP (cold control pressure) is too high at 3.4 bar/49.0 psi. that is already WCP (warm control pressure). The spec for WCP is 49~55 psi. and you are missing the capability for enrichment. You have no RICH mixture capability with your out of spec WUR.

[ChuckleBeast]

Originally Posted by theiceman

have you considered it could be your CDI unit failing under load ?

This was it, fixed the wire and swapped to a MSD streetfire. Running great now.

[mikedsilva]

Originally Posted by ChuckleBeast

This was it, fixed the wire and swapped to a MSD streetfire. Running great now.

Glad you found it.

Was the cdi unit still ok? or did you do both fixes at the same time?
This video below is me finding another green wire culprit!

[ChuckleBeast]

Originally Posted by mikedsilva

Glad you found it.

Was the cdi unit still ok? or did you do both fixes at the same time?
This video below is me finding another green wire culprit!

Fixing the Green wire didn't solve the problem. The original Bosch CDI was still giving a buzz but once I put in the streetfire it started right up and was running much smoother.

[mikedsilva]

Nice work. That break in the green wire would have eventually failed so good to change it anyway.
You can send your old CDI to Bob Ashlock - I think he buys the cores to refurb and sell on.

[mysocal911]

Originally Posted by mikedsilva

Nice work. That break in the green wire would have eventually failed so good to change it anyway.
You can send your old CDI to Bob Ashlock - I think he buys the cores to refurb and sell on.

This company buys cores for $350; https://www.systemsc.com/products.htm

[ChuckleBeast]

Any recommendations on where to get rebuilt? I got three other SC year cars and I like the idea of a spare.

[mysocal911]

Originally Posted by ChuckleBeast

Any recommendations on where to get rebuilt? I got three other SC year cars and I like the idea of a spare.

Check the link in post #11. They have 30+ yrs rebuilding CDIs.