86 930 Running lean
 

Hello all, I'm working through some issues with a US spec 86 930. The car is stock, and has been runing great. While cruising down the highway, it began to run rough below 3k RPM, over about 3200 it ran fine at light load.

The car will barely idle, and backfires with throttle application.

With slight pressure on the metering plate to richen the mixture, the car smooths out at idle and runs great.

CIS pressures are below:

Temp: 12C
System pressure: 6.8 Bar
Cold control: 2.0 Bar
Warm control: 3.9 Bar

Checked for vacuum leaks both with propane and pressurizing the intake, nothing found.

Could this be a fuel distributor issue? Everything else appears to be checking out just fine, but obviously I'm missing something!

Thanks!

Dumb question are both fuel pumps working a nd both red relays.

Yes, both pumps and relays functioning.

Kinda hard to diagnose over the phone as there can be a multitude of things that can cause this but here are some checks and info. With KOEO disconnect fuel pump safety connector on the back of the air flow meter and make sure the frequency valve is running (audible test). Passing that start car and do a duty cycle % test at the test plug on the relay plate, looks like a round relay plug connector. I don't have the pin diagram or the spec off the top of my head but should be easy enough to find. I want to say at idle the duty cycle should be in the 55% range.

Have you tried disconnecting the O2 sensor, if so what happens. Have you looked at the timing, does not sound like thats the issue but you need to start with basics of timing and fuel mixture.

On the 86-89 USA 911 turbos the lambda system runs closed loop up to 3000 rpm or a 65 degree throttle angle, once you pass either of those threshholds it goes in to open loop so if the engine runs ok past 3000 rpm it sounds like something is leaning it out during closed loop (O2 sensor ?)

Again kinda hard to diagnose without being there or lots more info. Hope this little bit helps.

Richard

Moderator please delete

One vacuum leak source that is difficult to check for with the methods you described (due to a closed throttle plate), and can cause the running issue you have, are loose or cracked injector blocks. For starters, put a wrench on each of the nuts (not like THAT! :eek:) and see if they can be tightened some. Most are pretty accessible with a normal-style wrench, except for a couple of them (such as the ones beneath the air metering assembly) which you might need to trim down a wrench for those.

Great idea, I will see how they feel with a wrench tomorrow. Interestingly, the fuel pumps do not shut off with the key on and the engine not running. When I did the fuel pressure tests, I had the metering plate sensor unplugged so the pumps would run.

After plugging it in, the pumps continued to run. I wonder if I have some resistance in the metering arm or the distributor piston near the bottom of the travel.

^^^

One thing that can cause the fuel pumps to run with the key on is a bad contact with the pins of the yellow relay (people call it the overboost relay, but it really isn't - I think the parts manual refers to it as the air metering relay). Mine was doing that and I could get the pumps to shut off by wiggling that relay in its socket. I first tried cleaning the pins and socket, but that didn't work, so took the cover off the relay and saw that two of the pins had cracked solder around them, so I reflowed the solder with a small-tipped iron and all good.

Unplug Lambda and manually set idle mixture to ~13.0:1 AFR then see what happens. Most likely culprit. Next would be the air meter assembly. This change happened abruptly which is indicative of these two issues.

Thank you for the detailed info, I will do some more digging tomorrow and report back. I failed to mention the O2 sensor is, and has been unplugged.

FWIW I've never noticed any difference running a regular relay here. I run a standard automotive square relay there now. No need to spend $ on a new yellow one afaict

How does the engine run on boost?
Does it stumble and run too rich or is all as normal?

It seems to run better as the boost starts, but I did not push the car since it was behaving like it was lean. I'm thinking I may throw the wideband on it and log some AFRs if I don't come up with anything else obvious. It runs so poorly down low, I have not driven it

Something has obviously changed, so I don't want to just "tune it away" without finding the root cause.

For what it's worth, checked all spark plugs and timing, everything is normal there.

I will spend some more time in the garage tonight and let you know what I find.

I had a few minutes to run down to the barn and do a quick test on the frequency valve. In my mind, I had ruled it out since the O2 sensor is disconnected. Low and behold, it is NOT buzzing with the key on. I did not have enough time to measure anything electrically, but I did lay my hand on the relay (real scientific, I know ;) ) under the seat, and it is clicking.

If the valve has failed, or is not receiving a signal at all, this would create a lean mixture, or 0% duty cycle, correct? It's been a long time since I've been this deep :)

I believe that is correct. s/b ~50% duty cycle without input afair

Yes very lean condition as the freq. valve is not dumping secondary control pressure from the fuel distributor.

Richard

[QUOTE=RT930turbo;11086577] Low and behold, it is NOT buzzing with the key on.

Were the pumps running, if the fuel pump are not running the freq. valve wont be activated either.

Richard

[QUOTE=porschyard;11086704] Yes, pumps were running. I corrected the issue with the pumps running all the time, the yellow relay socket was loose. After snugging the pins in the rubber socket, the metering plate switch now works correctly.

I did verify I have no voltage at the frequency valve, so I'm going to chase that rabbit up stream to determine where the issue may be. Does anyone have a pinout of the diagnostic connector in the engine compartment?

I do get 12V with the key on at the top pin, but nothing anywhere else.

Fixed
 

Sometimes you just have to get back to basics. :rolleyes:

Frequency valve is humming away, and it runs like a dream. After I swapped this relay. http://forums.pelicanparts.com/uploa...1604358799.jpg

Where was the black relay as i have had problems with my car.