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LJet, O2 sensor - bad running

10870 Views 59 Replies 12 Participants Last post by  Gubi
<I pulled this out of my Newbie thread to get a little more visibility>

I have done a ton of work on the car - you can check the other thread for all the details.

The issue I am trying to sort now has to do with the O2 sensor.

The car was running poorly. Stumbling, surging/dying at 1/2 throttle, not idling - not driveable. At the suggestion of a board member, I disconnected the O2 sensor, and viola 98% of the issues resolved. Car ran much, much better w/o the O2 sensor connected.

So, I bought a new O2 sensor and an 02 sensor special socket tool. I replaced the sensor and this is what I get:
  • With the sensor connected, I get the same old bad running condition. Stalling, not taking quick idle, surging...
  • With the sensor disconnected, it seems to run great.

This behavior started right off the bat. Put the new sensor in, plugged it in, started the car - car ran really bad. Disconnected the sensor - car ran great. Connected it, car ran bad... etc...

So even with a new O2 sensor it runs bad.

As always - I am open to ideas and thoughts on the cause of this.

Thanks as always
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Find the grounds on the passenger side engine block and clean them up. Replace any loops that have wires tearing out of them. I sanded each brass loop and the place they attach to, then cleaned the bolt...this got my intermittent wipers back :p

Have you cleaned other grounds by the computer?
Hi John: I have been through the entire LJet diagnostics routine - including cleaning or replacing all grounds and cleaning the connector for the FI ECU.

I have looked all around both ECUs and do not see any ground straps. Am I missing something?
You're absolutely sure you don't have any air leaks? Especially at the four rubber hoses connecting the plenum to the intake runners?
The "grounds" are attached to the rear of the cylinder head on the passenger side. Our '85 spider has 4 such rounds (black wires).

We did a complete rebuild on our spider and are having similar issues with the O2 and .....
I am 99.9% sure there are no air leaks. All new hoses and plenum connectors. And I tried Eric's method of putting a thick plastic bag on the MAF opening and then reconnecting the big hose. Then I disconnected one of the smaller hoses. I blow into it while closing off where it came from and it all seems air tight. (like the smoke test, w/o the smoke).

Would an air leak make it run really, really bad with the O2 sensor connected, but really pretty good when the O2 connector is not connected?

I have cleaned all grounds.
Would an air leak make it run really, really bad with the O2 sensor connected, but really pretty good when the O2 connector is not connected?
Er, possibly?

I mean, with the O2 sensor disconnected, the L-Jet goes open loop. This means it just uses a lookup table of how much fuel to add based on the various sensors and doesn't use the O2 sensor to adjust fuel.

So I could imagine a situation where if you had an air leak somewhere weird like one intake runner, it could cause three cylinders to go super rich with the sensor connected but it would run okay-ish with it disconnected. Or if you had one injector that wasn't spraying right or something, that could do the same thing. But I'm kind of guessing here.

The L-jet guide has some instructions for using the O2 sensor voltage output as a rudimentary AFR meter. Suggest doing some testing like that: it'll tell you what the mixture looks like with the sensor connected/disconnected and may give some clue of the problem.
OK, thanks. I see the instructions on the LJet page. Will give that a shot.
Just a guess - does your car have the altitude compensator? It would be located near the FI computer - under the shelf behind the seats. I have no idea if this is possible or not but I am wondering if a wonky altitude compensator could cause the symptoms you describe. If your car does not have the altitude compensator then I think there is a jumper wire to add, move or remove - perhaps that is the problem?

Or maybe not...
It does have one.

I'll check the resistances again (don't remember the exact results from the first time I did this).
I went through this exact problem, but with Kjet w/lambda on a BMW engine I built. I was never able to get it to idle well with O2 sensor plugged in, so I just tuned the mixture while running open loop, and used the output from the O2 sensor to dial in the mixture (and a digital multi-meter, this is what you should do to check the mixture). I passed emissions just fine then. Early on I thought the O2 sensor problem was because I had a Stahl header and the sensor was in the collector and the exhaust gasses weren't hot enough to drive the O2 sensor, but over the course of 5yrs the problem got progressively worse. I narrowed the problem down to a bad ECU, or a fault in the engine wiring harness.

This little story probably isn't much help :wink2: Ljet is way more advanced than Kjet, but the lesson could be in there somewhere. I know that you said that you have tried two different ECU's with same results. There might be something grounding out or not grounding, when it comes to the O2 sensor feedback circuit. In my case, I had crossed battery cables once or twice, and I bet that damaged the KJET ECU. Also, I re-routed the engine wiring harness so that the ugly grey plastic sheith came up from under the intake runners, so that stressed all of the connections on the top of the motor. I could have introduced a problem there, also.

Good luck and keep posting up...
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If the intake side is good I suppose it could be an exhaust leak upstream of the sensor. You hear any loud ticking or see any soot stains in that region?

Anyway, see what the sensor voltages look like with the sensor plugged in and not and then report back.
Will do... I gotta admit that I can turn a wrench, am dogged and not too dumb... but electronics is not my thing.

When you say "the sensor plugged in and not" - what exactly do you mean?

Not to totally expose my ignorance, but where do the red and black leads on the multimeter go when plugged in?
Red lead goes to the wire on the sensor, black goes to any convenient ground.

You want to measure voltage both with the sensor plugged into the harness and unplugged. With it plugged in, just clip your voltmeter to the terminal with it plugged into the harness. With it unplugged, clip your voltmeter to the sensor side of the plug (and don't let the harness side touch anything and ground out).

Engine needs to be hot when you do this. Sensor needs to be hot, too, so usually good to rev it up a bit for like 30sec before measuring.

>.45V is rich, <0.45V is lean. At idle with the sensor plugged in it *should* oscillate between 0.1-0.8V or so (bouncing regularly back and forth across 0.45V). With the sensor unplugged it should be steady somewhere >0.45V (slightly rich).
Ah, thanks. All you guys are greatest!!!
OK - here are the results... not sure what it tells me, nor what to do about it.

Warmed up, at idle.

Connected: Bounces between .22V and .00V
Disconnected: Steady at .60V

Next steps?
And, Eric: the Alt. Comp...

The Ljet diagnostic page states: Get out the OHM meter and check resistance between 2 and 3. The resistance there should be 2000-3000 ohm. Now measure between 1 and 2. The resistance value will depend on you altitude. If you are between sea level and 4000 feet the value should be 500-4500 ohms.

I've got:
2-3: 2400 ohm
1-2: 1400 ohm

So... seems in spec.
Connected: Bounces between .22V and .00V
Disconnected: Steady at .60V
Okay, you're lean at idle and it's apparently hitting the trim limit for adding more fuel. Open loop you're on the rich side (where you should be) but not very far. Which is semi-useful information, at least.

It's usually pretty tough to get an FI car to run lean without an air leak. I'm not going to pretend to have any brilliant thoughts, but stuff to check:

- Check again for air and exhaust leaks (sorry!). Plenum hoses can go bad. Also the hard plastic intake section across the engine can crack where the allen screws hold it down and give hard-to-find air leaks. Exhaust leaks you should hear as a ticking noise or see soot.

- On top of the usual locations, check the brake booster hose, the hose to the manifold pressure sensor, and the hose to the carbon canister. Clamp them off one at a time and see if it improves running

- Oil fill cap and dipstick are tight? Hoses to air oil separator (especially the two little ones underneath) hooked up and tight?

- You checked your temperature sensors (coolant in the engine and air in the AFM) with an ohmmeter, correct?

- Bad injector? If you disconnect the spark plugs one at a time, does it run equally bad for each? (don't do this too long...not good for the cat to dump a lot of unburned fuel into it)

- Is the CO adjusting Allen screw on the AFM still plugged or does it look like someone may have messed with it?
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Thanks. I will re-verify this all tomorrow or Sat.

On the brake booster, I unplugged that hose the other day after running the car for a bit and got a huge vacuum noise, so I'm very sure that is not leaking.

The AMF has been messed with, but seems to be working. (Photo below)


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The plug (silver, lower right) is what I was talking about. It covers the idle CO adjustment screw, which is an air bypass. I *think* it looks like it's still covered?

If someone took off the black cover and farked with the spring tension that's a whole other kettle of fish.
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