L-jetronic Troubleshooting, Runs Rough and Runs Rich Options

[BENBRO02]

It starts okay when cold and only has a slight miss but as it warms up it starts to miss more frequently, bucks, and if hot and idling may cut off completely. If I'm on the highway it may lose 20 mph before it starts running well enough to maintain speed or accelerate. I have recently had the following replaced:

fuel tank
fuel pump
fuel lines and hoses
fuel sending unit
vacuum hoses
large and small injector seals

The shop also checked the vacuum advance and cleaned the injectors.

I emptied my bank account with the repairs so now it's time for me to find the problem. Any Ideas? The car has a Pertronics ignition.

[jim_hoyland]

I would check the FI harness and a couple of the connections 1st;
1. Is the connector going to the left side of the dual relay firmly attached ? I used to have the FPump wire back off it’s spade and cause havoc.
2. Are the wires in the resistor pack all connected, look for a loose soldered wire barely making contact
3 does your FI have electrical tape repairs ? Could be a sign someone resolved a wire or two and it’s coming loose
BTW: I have a L-Jet FI Barnes listed in the classifieds

Good luck on this

[BENBRO02]

I would check the FI harness and a couple of the connections 1st;
1. Is the connector going to the left side of the dual relay firmly attached ? I used to have the FPump wire back off it’s spade and cause havoc.
2. Are the wires in the resistor pack all connected, look for a loose soldered wire barely making contact
3 does your FI have electrical tape repairs ? Could be a sign someone resolved a wire or two and it’s coming loose
BTW: I have a L-Jet FI Barnes listed in the classifieds

Good luck on this

Thanks for the advice. I'll try those and see what happens. I'm not sure I understand exactly what your saying though. I'm technically challenged and just learning where stuff is. Is it possible some thing is plugged in wrong? I did go check out your wiring harness and thought to myself that is was expensive until I saw a new one from Bowlsby is $752 plus a core charge. Now your used one in good condition seems inexpensive.

[Cupomeat]

I'd do whatever you can to make sure there is no vacuum leaks. Places I thought I had fixed before were the Valve cover gaskets (Sometimes the top of the gasket sags to allow a leak), oil filler cap gaskets AND the gasket between the oil air separator and the case. Try smoke or just using starter spray while running. Looks for something that may move when running.

Now, since you are saying there is intermittency, I'd agree with Jim and look at wiring as well.

Good luck!

[BENBRO02]

I'd do whatever you can to make sure there is no vacuum leaks. Places I thought I had fixed before were the Valve cover gaskets (Sometimes the top of the gasket sags to allow a leak), oil filler cap gaskets AND the gasket between the oil air separator and the case. Try smoke or just using starter spray while running. Looks for something that may move when running.

Now, since you are saying there is intermittency, I'd agree with Jim and look at wiring as well.

Good luck!

Where is the oil air separator located?

Thanks

[jim_hoyland]

Are you familiar with the dual relay and resistor pack ?
The dual relay is on the passenger side and has two connectors from the FI harness. The relays seldom go bad, but a loose connection at the base should be checked out

The resistor pack is usually located near the base of the battery, it’s an aluminum shell with 4 ceramic resistors. The FI harness plugs into the resistor pack; check that connection, then verify each of the ceramic resistors has a wire soldered firmly.

I mention these two; others will chime in with more things to check out.

[BENBRO02]

I would check the FI harness and a couple of the connections 1st;
1. Is the connector going to the left side of the dual relay firmly attached ? I used to have the FPump wire back off it’s spade and cause havoc.
2. Are the wires in the resistor pack all connected, look for a loose soldered wire barely making contact
3 does your FI have electrical tape repairs ? Could be a sign someone resolved a wire or two and it’s coming loose
BTW: I have a L-Jet FI Barnes listed in the classifieds

Good luck on this

I just drove the car around the block (about 5 miles) and although it is missing I never felt like I was in danger of being stranded. It's cold out so I would say it definitely runs better when it's cold out. I also looked in the engine compartment and inspected the wiring which in my opinion looks pretty good.

[BENBRO02]

Are you familiar with the dual relay and resistor pack ?
The dual relay is on the passenger side and has two connectors from the FI harness. The relays seldom go bad, but a loose connection at the base should be checked out

The resistor pack is usually located near the base of the battery, it’s an aluminum shell with 4 ceramic resistors. The FI harness plugs into the resistor pack; check that connection, then verify each of the ceramic resistors has a wire soldered firmly.

I mention these two; others will chime in with more things to check out.

This helps a lot. I had no idea where these things were.

Thanks

[djway]

First since you are challenged, as we all were and are to some extent, find every L jet manual out there, download, print and read. Over and Over and Over. I had never worked on FI before and I discovered bad wires, multi vacuum leaks, a bad potentiomoter board, weak CHT sender, air density sensor was corroded and the flap spring had been moved I assume to adjust for all the air leaks. It started as the motor cutting out every so often and belching smoke. Each time I found a problem and fixed it the car ran worse until I found all the problems.
I think I started with checking continuity on all FI related wires. Then I went to vacuum leaks and found a torn plenum and a torn tube end. I did a leak test of the injectors. Checked the AFM board to adjust the wiper but it had cracks so I replaced that. Checked the cold start valve for proper operation. Checked and cleaned the full throttle position mechanism. I then checked the resistance of the CHT and it was not in spec and they were hard to find before 914 Rubber started selling them. I then checked the AFM air density sensor and found the resistance was way off so I opened it up and cleaned the contact and bingo. After that I took an AF Meter and made an exhaust extension to hold the O2 bung and jumped on the freeway to find the proper stoichiometery for the mixture. Kept pulling off the freeway and moving the spring until I got it right at 3000 rpm. She now runs like a champ after barely limping home the day of purchase.
I knew NOTHING when I started. The L Jet service manuals have a check list of what to do. Start at the beginning and methodically work your way through one step at a time. Don't skip steps. If you are satisfied after completing each individual step the last thing would be the brain and I did not look into testing that.
You can do it. Read, learn, go slowly, methodically and you can get there.

[rhodyguy]

When warm, do you have a normal idle speed or a struggle to keep the engine idling at all?

[Dave_Darling]

What does the tach do when the engine starts missing? If the needle jumps all over, you definitely have an ignition system problem. (A lack of jumping around doesn't rule out ignition, so this is can only confirm one possibility but it will not deny any.)

Intermittent problems are usually electrical. If it runs fine for a while then runs crappy, there's a very good chance that the problem is an electrical connection.... somewhere. If it runs crappy all the time, it could be a lot of different things.

Do you have an IR thermometer? If so, run the motor for a while and then zap each exhaust tube near the head. If you find one is rather cooler than the others, that says you have one cylinder that isn't firing or is intermittently firing. If the pipe is dead cold, you have a "dead hole" and you can start checking stuff for that one cylinder.

You may be able to tell large variations in temperature by holding your hand NEAR each pipe in turn, but DO NOT TOUCH THEM unless you enjoy second-degree burns.

Good luck!

--DD

[timothy_nd28]

I hate intermittent issues, but it sounds like you can duplicate the issue easily once the engine has warmed up.

If I were in your shoes, I would warm the engine up till it starts to miss. At this point, I would then attach a timing light to any spark plug wire and stare at the strobing light as your engine is stumbling. Then I would move the timing light to the other three remaining plug wires.

You should see a consistent strobe pattern of light, any pauses in this pattern would indicate a ignition problem vs fuel. I think this would be the easiest to differentiate between spark and fuel, which should be the first step in any diagnosis.

[JeffBowlsby]

Links to 4 different L-Jet diagnostic manuals can be found here:

http://bowlsby.net/914/Classic/TechNotebook.htm

[Cupomeat]

I'd do whatever you can to make sure there is no vacuum leaks. Places I thought I had fixed before were the Valve cover gaskets (Sometimes the top of the gasket sags to allow a leak), oil filler cap gaskets AND the gasket between the oil air separator and the case. Try smoke or just using starter spray while running. Looks for something that may move when running.

Now, since you are saying there is intermittency, I'd agree with Jim and look at wiring as well.

Good luck!

Where is the oil air separator located?

Thanks

Sorry for the delay, it is the stack below the oil fill. There is a cork gasket between it and the block that can sometimes leak air. Good luck!

[BENBRO02]

When warm, do you have a normal idle speed or a struggle to keep the engine idling at all?

A struggle to keep the engine idling.

[BENBRO02]

Links to 4 different L-Jet diagnostic manuals can be found here:

http://bowlsby.net/914/Classic/TechNotebook.htm

Thanks, I had already found one and the others will come in handy. I can't wait to become an L-jetronic expert.

[BENBRO02]

I hate intermittent issues, but it sounds like you can duplicate the issue easily once the engine has warmed up.

If I were in your shoes, I would warm the engine up till it starts to miss. At this point, I would then attach a timing light to any spark plug wire and stare at the strobing light as your engine is stumbling. Then I would move the timing light to the other three remaining plug wires.

You should see a consistent strobe pattern of light, any pauses in this pattern would indicate a ignition problem vs fuel. I think this would be the easiest to differentiate between spark and fuel, which should be the first step in any diagnosis.

Excellent idea, thanks

[BENBRO02]

What does the tach do when the engine starts missing? If the needle jumps all over, you definitely have an ignition system problem. (A lack of jumping around doesn't rule out ignition, so this is can only confirm one possibility but it will not deny any.)

Intermittent problems are usually electrical. If it runs fine for a while then runs crappy, there's a very good chance that the problem is an electrical connection.... somewhere. If it runs crappy all the time, it could be a lot of different things.

Do you have an IR thermometer? If so, run the motor for a while and then zap each exhaust tube near the head. If you find one is rather cooler than the others, that says you have one cylinder that isn't firing or is intermittently firing. If the pipe is dead cold, you have a "dead hole" and you can start checking stuff for that one cylinder.

You may be able to tell large variations in temperature by holding your hand NEAR each pipe in turn, but DO NOT TOUCH THEM unless you enjoy second-degree burns.



Good luck!

--DD

Thanks it's great hearing all these ideas.

[BENBRO02]

First since you are challenged, as we all were and are to some extent, find every L jet manual out there, download, print and read. Over and Over and Over. I had never worked on FI before and I discovered bad wires, multi vacuum leaks, a bad potentiomoter board, weak CHT sender, air density sensor was corroded and the flap spring had been moved I assume to adjust for all the air leaks. It started as the motor cutting out every so often and belching smoke. Each time I found a problem and fixed it the car ran worse until I found all the problems.
I think I started with checking continuity on all FI related wires. Then I went to vacuum leaks and found a torn plenum and a torn tube end. I did a leak test of the injectors. Checked the AFM board to adjust the wiper but it had cracks so I replaced that. Checked the cold start valve for proper operation. Checked and cleaned the full throttle position mechanism. I then checked the resistance of the CHT and it was not in spec and they were hard to find before 914 Rubber started selling them. I then checked the AFM air density sensor and found the resistance was way off so I opened it up and cleaned the contact and bingo. After that I took an AF Meter and made an exhaust extension to hold the O2 bung and jumped on the freeway to find the proper stoichiometery for the mixture. Kept pulling off the freeway and moving the spring until I got it right at 3000 rpm. She now runs like a champ after barely limping home the day of purchase.
I knew NOTHING when I started. The L Jet service manuals have a check list of what to do. Start at the beginning and methodically work your way through one step at a time. Don't skip steps. If you are satisfied after completing each individual step the last thing would be the brain and I did not look into testing that.
You can do it. Read, learn, go slowly, methodically and you can get there.

Wow, very detailed and helpful. Thanks for taking the time to help.

[Mueller]

On my last 1.8 I made plugs that go into the boot to help eliminate vac. leaks.