I have a 1974 Wagon that I have a constant problem with the FI that I can't figure out. Unlike the D-Jet there is not a lot of trouble shooting information on the L-Jet.
Here is the situation:
Car idles poorly and "hunts."
Car is running too rich, black smoke comes out of the tail pipe. Copious amounts when I hit the accelerator.
Car has poor power.
What I have checked:
- CHT sending unit is hooked up. In fact I noticed a cracked wire and I just replaced it to be sure.
- All four cylinders are firing.
-Timing correct
- Fuel pressure reads about 38 psi when cranking and drops to about 32-34 psi when idling. It was my understanding that the correct idle fuel pressure is around 28 psi for l-jetronic.
- I can find no vacuum leaks , particularly on the hose that goes to the fuel pressure regulator. I have tried carb cleaner and a unlit propane torch, no leaks found. All hoses are relatively new and in good shape. I have verified that the hose going to the fuel pressure regulator is holding a vacuum when running.
- I know this is stupid but I replaced the fuel pressure regulator thinking it was bad. No change in rich condition or fuel pressure.
- For a while the problem was intermittent. Whenever that happens I always think electrical.
So where do I go from here?
- does anyone know the correct vacuum pressure that should be going to the regulator? If I knew that I could test it.
- Am I looking at the wrong thing? What should I test here? I wish I have a really good l-jet book. I do have a complete original set of 914 shop manuals (about 10 binders), I wonder if there is a section on troubleshooting the l-jetronic.
Anyone have any ideas?
Thanks,
Kevin
L-Jet Woes
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ubercrap
- Posts: 1394
- Joined: Sun Apr 18, 2004 8:00 pm
Maybe I am way out of line, given my extremely limited experience compared to most of you guys (I haven't driven my 412s much at all), but when I was replacing some fuel hoses in the engine compartment of my '74 wagon, I broke some of the extremely fragile wires going to the series resistors for the injectors. I'm not exactly sure what the effect this has on the system, but the car would still run, just poorly and with no power. I can't remember if it was running really rich, though.
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raygreenwood
- Posts: 11910
- Joined: Wed Jan 22, 2003 12:01 am
Ok....a couple things. Yes, your fuel pressure is too high.
Things that could cause that with L-jet:
A vacuum leak or a restriction at the return line somewhere...causing the fuel pressure to be elevated artificially.
Also, check to make sure your throttle is closing all the way.
If you can find one of those mighty vac hand pumps, you can test to see if the regulator is even capable of giving you pressures low enough for idle. You need to hook up vacuum and see if it can bring it down to about 28 psi.
If it does...then the problem might either be that you have the wrong regulator (too strong of a spring for your vacuum level) ...or...too low of a vacuum level if the regulator is actually correct.
IfYou end up with 15Hg of vacuum on the pump and the regulator still will not drop below 34 or so...then you have either a defective regulator or a restriction in the return line.
Also...what pump are you running? It would be exceedingly rare...but it is possible that if you have a humongous pump...it could be over-pressurizing. Doubt it though.
Also...make sure you have the pressure going in the right direction through the regulator.
Lastly...some potential sources of vacuum leaks that are very hard to catch.
Injector seals
runner boots
the o-ring between the TB and the plenum
Runner to head gaskets
valve cover gaskets
hope this helps. Ray
Things that could cause that with L-jet:
A vacuum leak or a restriction at the return line somewhere...causing the fuel pressure to be elevated artificially.
Also, check to make sure your throttle is closing all the way.
If you can find one of those mighty vac hand pumps, you can test to see if the regulator is even capable of giving you pressures low enough for idle. You need to hook up vacuum and see if it can bring it down to about 28 psi.
If it does...then the problem might either be that you have the wrong regulator (too strong of a spring for your vacuum level) ...or...too low of a vacuum level if the regulator is actually correct.
IfYou end up with 15Hg of vacuum on the pump and the regulator still will not drop below 34 or so...then you have either a defective regulator or a restriction in the return line.
Also...what pump are you running? It would be exceedingly rare...but it is possible that if you have a humongous pump...it could be over-pressurizing. Doubt it though.
Also...make sure you have the pressure going in the right direction through the regulator.
Lastly...some potential sources of vacuum leaks that are very hard to catch.
Injector seals
runner boots
the o-ring between the TB and the plenum
Runner to head gaskets
valve cover gaskets
hope this helps. Ray
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wildthings
- Posts: 1171
- Joined: Thu Jun 03, 2004 12:42 am
If your new regulator gives the same pressure as your old I would question your gauge.
I would guess that the rheostat strip in your AFM is worn. Pop the lid off and take a peek. Look at the track for the wiper and see if it is nice and smooth and not worn looking. The Bentley for a Vanagon has pretty good trouble L-jet shooting information, so I would think that the one for a Bay bus would be good as well. You can pretty much go through the entire trouble shooting sequence in only 10 minutes or so.
I would guess that the rheostat strip in your AFM is worn. Pop the lid off and take a peek. Look at the track for the wiper and see if it is nice and smooth and not worn looking. The Bentley for a Vanagon has pretty good trouble L-jet shooting information, so I would think that the one for a Bay bus would be good as well. You can pretty much go through the entire trouble shooting sequence in only 10 minutes or so.
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vankstwer
- Posts: 4
- Joined: Sun Nov 05, 2006 2:00 am
L-jet woes
you guys are pretty much right on with your advice. for a while i was baffled about D and L-jetronic systems on the vw's but being an auotmechanic by trade i knew better than taking an easier route with carbs. so i feel you confusion with your problem. I found on the "type4.org" web site to be pretty insightful. just go to "manuals.type4.org" i have found it to be very useful when coming to troubleshooting your problems with your l-jet system. Give em a try and see what they got.
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Bowman74
- Posts: 51
- Joined: Mon Nov 22, 2004 9:33 pm
I just wanted to post the resolution and thank everyone who offered advice.
I found two issues.
1) The clamp for the hose from the auxiliary air valve to the intake was a little loose. I don't think that was my problem but I wanted to mention it as it may have had a causal effect on the higher than normal fuel pressure.
2) The flap in the air flow meter was sticking partially open (sometimes). This resulted in the wrong position being sent to the CPU; bad things followed such as too much fuel being injected at idle. It also explains the intermittent nature of the problem slowly leading to eventual failure (when it finally really stuck). I used some carb cleaner to remove the surface corrosion and then lubed up the joint a little bit. I moves much more freely now but I suspect I'll be taking it back off at some point to take another run at cleaning it.
Car now runs fine. Idling fuel pressure between 28-30 PSI. Copious amounts of black smoke gone. Power is back.
Once again I'd like to thank everyone who helped me out. Next project, making extenders for Audi 4000 strut cartridges to use in my 412. Ray, don't you have a blueprint for those extenders hanging around somewhere?
Thanks,
Kevin
I found two issues.
1) The clamp for the hose from the auxiliary air valve to the intake was a little loose. I don't think that was my problem but I wanted to mention it as it may have had a causal effect on the higher than normal fuel pressure.
2) The flap in the air flow meter was sticking partially open (sometimes). This resulted in the wrong position being sent to the CPU; bad things followed such as too much fuel being injected at idle. It also explains the intermittent nature of the problem slowly leading to eventual failure (when it finally really stuck). I used some carb cleaner to remove the surface corrosion and then lubed up the joint a little bit. I moves much more freely now but I suspect I'll be taking it back off at some point to take another run at cleaning it.
Car now runs fine. Idling fuel pressure between 28-30 PSI. Copious amounts of black smoke gone. Power is back.
Once again I'd like to thank everyone who helped me out. Next project, making extenders for Audi 4000 strut cartridges to use in my 412. Ray, don't you have a blueprint for those extenders hanging around somewhere?
Thanks,
Kevin
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vankstwer
- Posts: 4
- Joined: Sun Nov 05, 2006 2:00 am
L-jet woes
Glad to hear you got er runing back to normal again. I own a 73' 412 sedan myself and it has that D-jetronic setup and when i was attemping to get it back runing again i started to have vacuum leaks from hell out of every where. I almost thought for a min there that the preasure sensor was just shot, but come to find out all of the original german vacuum hoses were just all dry rotted. so i started to change em all out and replaced em with just regular rubber vacuum hose. apparently just pushing them snug into place wasnt enough, it still was leaking somewhat to a degree. so i went and got some much better material (silicone vacuum hose) and it cleared it all right up. everything was tested and came out ok. runs like a dream. plus the benifit of the silicone hose is that its more resistant to heat than the rubber hose and it last three times longer. impervious to oil also, so no more brittle vacuum lines. the extra cost and effort is worth the hassle. i reccomend it to anyone that has a system like ours.
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raygreenwood
- Posts: 11910
- Joined: Wed Jan 22, 2003 12:01 am