Jetdriver69

From my last post, my 82 will occasionally idle at very low rpms (400 rpm and act likes it want to stall) after start even on a warm engine. After revving, the rpms will rise to about 12-1400 rpms and slowly go back to 750-800 rpm and idle normally.

If you restart after the problem occurs, the engine idles just fine.

Other times, the car fires right off, the AAV increases rpm to 1200 and then reduces to 800 like normal.

I replaced the fuel pump relay and I couldn't find any vacuum leaks.

I disconnected the battery ground for about an hour. Doesn't that force the O2 sensor to find a null setting?

After I did that, the car idled as normal.

I guess I need to order a fuel injection relay and test the AFM for proper resistance if the problem persists.

Could the AFM be in the first stages of a failure?

New fuel pump, injectors, injector harness, ignition components, green wire, ignition timing 25 BTDC.

Any suggestions as it is a very occasional problem and only very recently started to act up.

Thanks!

Lizard928

Check the FPR, and damper for failed diaphragms.
Get a FP gauge and check for fuel pressure leakdown after shut down.

yaskota

And- as another (but '84) L-Jet owner who experienced rather odd idle issues after replacing FP regulator (front) , FP relay, green wire, spark plugs and wires (and more)... check your Temp Sensor (2 IIRC) also towards front & near FP Regulator.

I say this though knowing that if I'm wrong, someone will kindly correct me as I am certainly not a mechanic! ;-)

The Forgotten On

Yep, temp two is on the water bridge near the FPR, it's the one with the amp plug instead of two spade connectors.

I don't think removing power to the ECU will do anything as our L-jet computers are analog unlike the digital LH which can be reset.

This makes them pretty bulletproof when it comes to reliability as their failure rate is very low.

Check the resistance on you AFM, it only takes a few minutes and can solve a lot of mystery running issues.

Unlikely but possible, your AFM also might be binding a little when you start the car and cause it to starve the engine of gas and air, you never know with these 30+ year old cars.

Jetdriver69

I checked the FPR and dampers, no fuel in the vacuum lines.

The fuel pump only has 5000 miles on it and I can hear it run, so I don't think that is it. I put a new FP relay in, but that didn't solve anything.

Checked the fuel injector harness, all connectors were on tight.

Temp II sensor is new, but will check the resistance anyway.

Could a O2 sensor going bad or having a loose wire splice cause these symptoms? What about a L Jet relay going bad?

I guess it could be the L Jet brains, but I read they rarely go bad. Would checking the impedance on the brain diagnose a faulty controller?

If it happens again, I guess will pull the AFM and check the impedance. May as well pull the AAV and check it too.

The AAV gave me some problems about a year ago, but testing the valve in the freezer a few times seemed to get it to work.

From what I read, if the decel valve was going bad, I would have a high idle not an occasional low idle. So I don't think that is it.

I have the idle switch disconnected to improve throttle response lag, but that was done over a year ago and I never had these symptoms.

The most annoying thing is that the problem doesn't happen all the time, only about every 5 or 6 starts. Doesn't matter if the engine is cold or warm.

Thanks

Jetdriver69

I discovered the barn door in the AFM was a bit sticky.

Cleaned it up a bit and lightly lubed the pivot point.

Seems to work better, no instances of low or rough idle today in 6 starts.

Could that have been the problem?

Thanks

Fogey1

Just to keep them linked for posterity -

Original thread (this thread):
https://rennlist.com/forums/928-foru...-problems.html
L jet idle problems

Thread with solution:
https://rennlist.com/forums/928-foru...ion-relay.html
L Jet fuel injection relay

Jetdriver69

OK I spoke too soon about completely eliminating the intermittent idle problems with my 82 5 speed.

From the prior post, I discovered the L Jet relay #16 was one its last leg, so a new one from Roger was installed. That seemed to cure everything, however the symptoms reappeared yesterday. Hard to restart after warming up and then runs fine and settles to normal idle after a couple of revs past 3000 rpm.

Today I removed the plenum and checked the rubber sleeves for pin hole leaks, none found. The rubber seal between the plenum and throttle body was also good.

Then while the plenum was off, I took out the decel valve. I have always suspected valve was sticky because of a occasional high idle while coming to stop light.

Well the short vacuum hoses coming off the throttle body to the decel valve were absolutely rotten. I could tear them in half with very little effort. I must have missed those lines when I replaced all the other vacuum lines last year or they looked "good enough" at the time.

Capped the throttle body off with a 3/4 and 5/8 rubber caps and no more decel valve.

While the AFM was out, I checked the ohm readings on the 5 connectors per the WSM sequence and all were in tolerances.

I popped off the plastic cover of the AFM to check the condition of the wiper track. As you can see from the picture, there is a very obvious wear track and from what I have read, a potential cause of all my idling and hard starting problems.

I found a 944 tutorial showing how to gently bend the wiper back a tiny amount so it rides on a fresh piece of resistive surface.

Put everything back together with a bit of silicon around the plastic cover and reinstalled.

I even reconnected the idle switch to see if that could have been a contributor to the rough idle.

Started it up and it ran great, great throttle response but it did have a bit of hesitation on the idle rebound. So the idle switch got disconnected again. That solved that issue.

The idle is rock steady and after a rev past 3000 it settles back to 800 very quickly with zero lag.

So in summary, I think my initial problem was a combination for 3 or 4 things.

1. Probably needed a new fuel pump relay and L Jet relay, but that wasn't the root cause of the problem.
2. I had a decent vacuum leak through the decel valve and/or the associated vacuum lines. That causes a slightly lopey idle, which is gone now.
3. The AFM wasn't not working at 100% due to the worn wiper track. After moving the wiper arm to non worn part of the track the throttle response is much better.
4. The idle switch did cause a lag on the idle rebound, disconnect that switch and idle is steady with quick recovery. Thank Mark K for that tip!

Hopefully this will be helpful to other L Jet guys as the symptoms were maddening and very intermittent.

I highly suggest everyone with an AFM to pull it out and check for wiper track wear. It is very easy to fix and only takes about an hour to include testing the AFM connections. Definitely makes a difference!

Ladeef

Thanks for this, I have the same issue with my 82. I"ll go down the road you went.