Oscillating sensor plate

[ninelevenick]

I've conquered my WUR and have adjusted the CCP to an optimal pressure. I have also been playing around with adjusting the WCP. Currently it's set at 40psi because my car ran very nicely with that setting a few weeks ago, but the car only ran well with that setting for a 10 mile duration or so. My car is an ROW with a 069 WUR, so I am aware that it should be set around 48 psi. Regardless, I can set my WUR's pressures, and it will hold. I give this detail because the WUR comes up when diagnosing fuel and air questions.

Anyway, I have been chasing a low rpm misfire for quite some time. From a cold start, the engine will start to misfire resulting in backfires through the exhaust. I have headers and a low restrictive muffler, so the backfires are very audible. The backfires occur during mid rpm throttle.

Things I've done:
Adjusted WUR
Checked coil - good
Compression check - very good results
Checked for vacuum leaks - replaced air box, seals, gaskets, etc. No detectable leaks
Verified that injectors spray evenly.
Serviced the ignition distributor
New ignition wires
Newer spark plugs, cap, and rotor
New fuel filter

I have to admit, that I have been messing with the AFR. Truthfully, I have been ever since I bought the car because of the issues it had from the beginning. Things are getting sorted. I have been tuning it by ear. Here's my latest observation: the sensor plate will sometimes start to oscillate very quickly if I adjust the AFR counter or clockwise, and sometimes it starts to oscillate if I blip the throttle. I can temporarily stop it from oscillating by slightly pushing up on it, blipping the throttle, or changing the AFR. Once it starts oscillating, the engine starts to really misfire.

One more thing: I have changed my spark plugs three times recently, and every single time #1 and 4 are carbon/soot gunked. The rest look good.




I have done some research, and I found a couple of old posts that point to the fuel distributor as being the culprit. I want to be more clear on this before I send it out for refurbishment.

There's two more things I will probably do: Check the fuel return lines near the tank and check the oil breather hose for holes. During the process of reinserting the engine into the car last September, the breather hose got caught between the engine tin and chassis.

I know this a lot of information. I wanted to make sure I covered everything I can think to eliminate possibilities. Thanks in advance for any insight you can give me.

[stlrj]

Judging from your plugs alone, I would lean the mixture by at least an eighth to a quarter turn and drop in a new set of plugs before firing it up.

Cheers,

Joe

[john walker's workshop]

Bad fuel distributor.

[gomezoneill]

Just my 2 cents the spark plug gap looks tight.

[ninelevenick]

Thanks Joe. I did just that. I haven't fired her up to see if it makes a difference.

John- I saw two other threads where you suggest the same. Without a doubt bad FD?

[ninelevenick]

Yeah, those plugs are gapped .28. I gapped the new set at .30. Do you think that's a good gap?

[john walker's workshop]

Originally Posted by ninelevenick

Thanks Joe. I did just that. I haven't fired her up to see if it makes a difference.

John- I saw two other threads where you suggest the same. Without a doubt bad FD?

Every one I've fixed turned out to be the FD.

[stlrj]

Originally Posted by ninelevenick

Yeah, those plugs are gapped .28. I gapped the new set at .30. Do you think that's a good gap?

I gap mine at 0.045"


Cheers,

Joe
87 Carrera

[ninelevenick]

Standard Bosch ignition, Joe?

[allaircooled]

I imagine one of those threads was mine. My car doesn't back fire though. The idle just surges when the oscillating occurs. Your mixture might be still off. Do only 1/8 turn adjustments. I haven't replaced my fuel distributor yet.

[stlrj]

Originally Posted by ninelevenick

Standard Bosch ignition, Joe?

Yes, nothing special, but it does like the bigger gap yet never skips a beat.



Cheers,

Joe

[ninelevenick]

Okay. I'll give it a shot.

[Sub8]

Originally Posted by john walker's workshop

Every one I've fixed turned out to be the FD.

John - what are the failure modes on the FD? Dirt / blocked or?

[T77911S]

JW is probably right, bad FD. I think it is related to the pick off circuit for the WUR inside the FD.
call larry at flowcontrol.

first off, you can set the CP anywhere then adjust the mixture to correct for the wrong CP and the car will run. perhaps not as designed but it will run.

your WUR has vacuum enrichment. make sure that works. the CP goes down with no vacuum.
you should be about 3.5abr with vac and about 2.8bar without.

here is what I would do especially since you are looking at a FD rebuild.

set the CP to spec.
set the timing and check the advance.
adjust the mixture rich until it the engine surges at idle and/or the RPM's drop down then come back up to idle after you blip the throttle. then adjust it lean until it no longer does this. leave the mixture alone.
swap injectors. 1 to 2 and 4 to 5.
see if you still have bad plugs on 1 and 4. actually they all don't look that great.
if the problem moves, you have bad injectors.
if not I would check the flow of all 6 injectors and compare the volume. if your FD has the adjustments AND 1 and 4 are flowing more fuel than the other 4, I would lean those 2 out.
NOTE: I only say this because it looks like your FD needs a rebuild.
check the volume at WOT and at what would be idle fuel flow.

if none of this fixes your issue send the FD to larry for a rebuild

[ninelevenick]

^^great advice. I will definitely run through your suggested steps. Thanks.

The photo I took of the plugs is kind of crappy. I had to use flash making the plugs appear darker than they really are.

[boyt911sc]

Nick,

Why spend numerous hours testing your FD and WUR? You have PMed me several times already and my suggestion to you is I will test them for FREE and send then back to you with the test results. Your expenses would be the postage back and forth. Or I could lend you a newly rebuilt and calibrated FD and WUR that you could use in your car. You are not obligated to buy them. They are just loaners to help members like you sort their CIS problem/s.

After you are done, send them back to me. No question asked. Your only expense is the postage sending the parts back to me. I will pay for shipping to you. BTW, I have provided hundreds of loaners to PP members and counting. I don't operate a business. I'm just an avid CIS troubleshooter and wanted to give back to fellow 911 CIS community. Thanks.

Tony

[ninelevenick]

Wow, Tony. That is very generous of you. I guess I'm a bit stubborn. If I send out my FD to you, I really want to be confident that it is the culprit. There's one or two things I want to try first. I'm very confident my WUR operates as it should.

[john walker's workshop]

Rebuilders may not be able to repair an oscillating FD, so be ready to pay a core charge. There's a little metal dome shaped piece at the very top of the main bore that somehow gets damaged and dimpled. You should be able to see the top of that piece, and the dimple by looking through the center port that goes to the WUR. With a light source. Rather inaccessible for repair.

[boyt911sc]

Originally Posted by ninelevenick

Wow, Tony. That is very generous of you. I guess I'm a bit stubborn. If I send out my FD to you, I really want to be confident that it is the culprit. There's one or two things I want to try first. I'm very confident my WUR operates as it should.

Nick,

Could you share with us the FD and WUR numbers on them? The information you gave me about the FD is alarming. I've never seen or heard one before. Interested to know the WUR too. BTW, did you measure your WCP (warm control pressure) with and out vacuum? The value you have is well off specification.

I felt good since I came home from Hershey Swap meet. I was lucky to bring home a box full of assorted Bosch WUR's for the price of scrap metals. These are the goodies:




Decide quick because I might be going with my wife to Australia again very soon or early summer for a couple of months.

Tony

[ninelevenick]

Sent you a PM, Tony.