Rough hot idle, lambda not working

[82DMC12]

Hey guys,

For the last couple of years my D has had a slightly rough hot idle with a sulfur odor from the exhaust. I'm trying to fix it now so I on DMCMW's advice I ordered a new Lambda probe. Here's what I have done so far -

First pulled all injectors at once and tested them. They all look good, no dribbling or leakage. They open at the same time. CSV is not leaking when the engine is hot.

Replaced O2 sensor. I have the stage 1 exhaust from the first time they were offered so I had to make a longer O2 wire. I covered the connections in heat shrink tubing and checked for continuity from the probe connector under the rubber cover to the female spade on the other end. The connection is solid.

I got the car warmed up and hooked up my dwell meter. I only have 6 cyl and 8 cyl scale on the analog meter. From my old notes I see if the lambda is unplugged it should be about 30 on the 6 cyl scale, and go up to around 38 if you press the WOT switch. It does this. When I plug the lambda back in, it doesn't swing at all. It stays at 30. I revved the engine up a while to clean the plugs and it just doesn't move. The FV is buzzing. I checked the Lambda ECU and found the ring ground under the huge black connector to be loose. I tightened that but it didn't help. If I unplug the ECU while the engine is running the dwell drops to 0. I assume that means the lambda relay must be good if the ECU is getting power at all.

So this must be my rough idle problem, also the sulfur smell and there was some brown oily residue on the injectors, I assume unburnt fuel. I need to get the needle swinging again.

What do you think I should do next?

Andy

[DMCMW Dave]

Check the ground wire at the back of the passenger side intake runner. It's ring lug bolted to the runner, then about 3 inches of wire then a black inline connector. Make sure it's plugged in and a clean connection. That's the ground reference for the O2 sensor and if disconnected everything goes wrong. Also check/clean the corresponding bulkhead connector pins.

Now do the tests in the Service Manual. I'm away from the shop this weekend so you'll have to find that yourself but its at the end of the fuel injection section.

Disconnect the O2 sensor. GROUND the lead to the ECU (not the O2 sensor wire) and watch the dwell meter. Now connect a 1.5 volt battery between the ECU (+ to ECU, - to ground) and watch the dwell meter. Grounding should drive the dwell to full (85) and with the battery it should go to low (usually about 10). If this works the ECU is connected and working correctly.

[82DMC12]

Thanks Dave - for the ring ground.... where does the other end of the wire start at? I mean, where does it come into the engine bay? I think I know which one you are talking about ; it has been relocated to the FV mount on the valve cover because the screw broke off the intake when I pulled my engine! I will try to check this stuff today and report back.

Andy

[Bitsyncmaster]

So when you press the WOT switch, do you see the reading change?

If not, my guess would be your WOT switch is always activated. With the FV buzzing your getting power to the lambda and FV. There also is a thermal switch that uses the WOT switch wiring.

[DMCMW Dave]

Thanks Dave - for the ring ground.... where does the other end of the wire start at? I mean, where does it come into the engine bay? I think I know which one you are talking about ; it has been relocated to the FV mount on the valve cover because the screw broke off the intake when I pulled my engine! I will try to check this stuff today and report back.

Andy

It goes through the bulkhead but it's not documented correctly on the Delman bulkhead diagram so I'm not sure what pin it is. The other end of it is the Lambda ECU. I'm guessing it's one of the black wires on the blue connector.

[82DMC12]

So when you press the WOT switch, do you see the reading change?

If not, my guess would be your WOT switch is always activated. With the FV buzzing your getting power to the lambda and FV. There also is a thermal switch that uses the WOT switch wiring.

Yes, when I press the WOT switch, the needle goes up to about 38 on the 6 cyl scale. When i release it, the needle goes back to straight up and down (32 or so on the 6 cyl scale)

I'll check the ground in a bit...

Andy

[Bitsyncmaster]

Yes, when I press the WOT switch, the needle goes up to about 38 on the 6 cyl scale. When i release it, the needle goes back to straight up and down (32 or so on the 6 cyl scale)

I'll check the ground in a bit...

Andy

Then your switch is good.

[82DMC12]

Then your switch is good.

Right. You might have misunderstood. I'm trying to troubleshoot why my dwell meter needle doesn't swing when the engine is warmed up. I have a new probe and there's no way the mixture is very far off because I had it set exactly a couple of years ago. I'm thinking there is a wiring problem and maybe Dave's answer will help me find it!

Andy

[82DMC12]

OK here's where I am now -

I used my multimeter and it seemed like the ring ground was a little flakey. I can tell someone did some splicing there before. I took all the old stuff out, cleaned the connection inside the plug (extremely dirty) and then retested and it was consistently good.

Then I started up the car. It's 62 degrees here so the car was fairly cold. It purred perfectly for about 2 minutes, then it was like someone fipped a switch and it started it get rough. The needle on the dwell meter was still steady at 30. As the car idled I was reading the shop manual and it said the O2 can't get a good reading until around 300 Celsius, so I figured it was still warming up.

I drove the car around until it was good and hot, then parked and checked the dwell meter again. This time it was actually swinging, but it was low in the range. I turned the CO screw a few degrees, plugged it, revved up, checked again, and tried that for a bit but I couldn't seem to get it in the right range. Too high or too low. I got it fairly close but it still didn't sound smooth.

I tried Dave's trick with grounding the ECU wire. When I do that, the needle swings to the top, then falls back down to 30. When I put the battery in-line, it swings to the bottom, then glides back up to 30. When I plug the O2 sensor back in.... still stuck at 30 even though the engine is hot.

Seems to be intermittently working.... what now?? Is it harder to tune a car with the DMCH headers and exhaust?

Andy

[82DMC12]

I decided to make a 3 min video to show what's happening with the dwell meter. You will see the meter is swinging all over, sometimes lean, sometimes rich, sometimes just stopping at 30 until I rev the engine again. Any ideas?

http://www.youtube.com/watch?v=NBnJ2mlcjP0

[DCUK Martin]

This is just an idea: can you pull off the vac feed to the distributor when it idles like that? any change?

[82DMC12]

This is just an idea: can you pull off the vac feed to the distributor when it idles like that? any change?

Martin,

Good thinking! I pulled the vac hose off the dist and the idle significantly improved. There is vacuum sucking through the hose that goes onto the distributor at idle, which doesn't appear correct. I verified with a timing light that the ignition is advanced (the crank mark is like 2 inches away from the reading plate).

I replaced the idle speed switch once a few years ago. I know it is clicking when the throttle closes.

What steps should I take to resolve this?

Andy

[Bitsyncmaster]

Good thinking guys. You are correct the advance should not be on when the idle switch is closed. Check that little connector on the vacuum solenoid. The idle switch is hard wired to the solenoid so broken wire or bad solenoid.

I would not think that would affect the lambda (dwell) though.

[82DMC12]

Thanks guys! Got it all sorted out!

The vacuum advance at idle was the key. I found that the solenoid was not clicking when I turn the throttle spool and let off the idle speed switch. The switch checked good with a meter, but upon unravelling some electrical tape on the wires I found a really bad connection that the 19 year old me must have done way back in the day :-)

I cut the bad stuff out, reconnected it, used heat shrink tubing and fresh tape, cleaned the microswitch, and now the advance stopped. Verifed timing with the timing light. All good now.

This time the dwell meter worked and it showed I was running lean. After a few tweaks and trips around the block, then retweaking, I have it very close. I think I will enjoy the car for a few days, go through a tank of gas, then recheck timing and CO. It was definitely running rich for quite a while, the O2 sensor was pretty fouled-looking and the injectors all had oily brown residue on them...

Man, I love the way this car sounds when it is tuned correctly!! No more embarrassing stumbling at stop lights :-)

Thanks again, guys!

Andy

[82DMC12]

Still having problems. I might have a flakey microswitch but I also want to check the ground. Anyone know where exactly the black ground wire from the microswitch ends up? What's up with the little thimble sized thing that's also on the ground side attached to the intake meter manifold?

Seems about 50% of the time the switch, which is adjusted properly with the set screw, is not making a ground which keeps the advance on at idle. Having trouble tracing all the wires!

[82DMC12]

Bumping to post my resolution. It turns out the idle speed microswitch I bought from a non DMCH-affiliated vendor is either broken or not electrically identical to the OEM switch. I bought an OEM switch from DMC Midwest, installed it yesterday, and my idle and ignition advance was back to how it's supposed to be.

So, if your idle is rough when the engine is hot... check your ignition advance. It might be activated!

Andy

[David T]

This is also a lesson to be very critical of ANY previous work, even work you have done. It is also a warning to be very careful when substituting replacement parts. Even though they may fit and look alright they do not always work like they should! Buying from the Delorean vendors gives some confidence the parts will be right for the application, most of the vendors use these very same parts in the service side of their business so they have experience with what works and what doesn't.
David Teitelbaum

[DBOZ]

I'm going to try this, Mine surges like that 50 percent of the time now. Last time I brought her to Dave, he didnt know what was goin on, but that was umm 3-4 years ago, and I was lets just say a 20yr old that didnt have his head on quite straight yet.
Thanks for all this advice, maybe mine will run good soon also!
Darrell

[dmc6960]

This particular problem can also easily be the electrical connector coming off of the solenoid. Its a very poor connector design. You'll need to look and test to see which component is the root cause.

Jim Reeve
DMC6960

[82DMC12]

This is also a lesson to be very critical of ANY previous work, even work you have done. It is also a warning to be very careful when substituting replacement parts. Even though they may fit and look alright they do not always work like they should! Buying from the Delorean vendors gives some confidence the parts will be right for the application, most of the vendors use these very same parts in the service side of their business so they have experience with what works and what doesn't.
David Teitelbaum

Those which know me know I am very anal about OEM parts and function. Unfortunately because my OEM switch failed I gave this modern replacement a try from a well known delorean vendor but I guess like usual, OEM would have been best!

Andy

[David T]

Then I guess this is also a lesson to be suspect of even new parts and even from Delorean vendors. You cannot just assume because a part is new it is good. Always troubleshoot and diagnose a problem no matter where it leads you, even if it leads you right back to the part you just replaced!
David Teitelbaum