Engine Shake at Idle

[Grover]

Ok, so I have had this problem before. The solution turned out to be a wire came off of the thermo connector under the air intake that relays to the lambda computer. It caused the mixture to run full no matter what.

Well, after many months of it being fixed, I started the car up to head back home last night and sure enough, the engine rocks and rumbles at idle. The mixture can be smelt as running rich. I unplugged the lamdba computer and it does create a change in the idle when unplugged and plugged back in. I also checked to see if the full throttle switch was stuck.

I am at a loss as to what else could cause a symptom like this. I doubt the wire came off again as I made sure that it would stay on. The frequency valve does buzz FWIW. It also tends to hesitate when taking off too until the RPM's are higher. Any tips would be helpful, thanks!

If more information or facts are needed, let me know.

-Steve

[Bitsyncmaster]

Could be a lot of things. So you need to put a dwell on the FV and verify it's running closed loop (swinging up and down) and properly centered.

The WUR could also make it rich. Does it improve after a long drive?

[Grover]

I am not at home until Thursday night, therefore no Dwell meter on hand. Though the systems are exactly the same as before when it would only run open loop. Highway MPG has also suffered.

It does not improve after a long drive at all.

[mluder]

Mine does the same - Rough idel and the occasional shimmy. Poor acceleration in the low RPMs and around 2200 it kicks in.

I have a really bad mix. Won't pass emissions and the duty cycle is all over the place and won't settle.

I replaced the ECU... that seemed to help for a bit but now I'm back to where I was before.

I only have 10,000 miles on the car so I don't think it's the lambda sensor.

I'm pretty confident I'm dealing with a vacuum leak but I'm not sure where.

So are duty cycle and dwell interchangeable? I was thinking about buying a meter that measures Duty but they are expensive. Harbor Freight (yeah I know) has one that measures dwell for $40. How do I hook it up to meassure and what am i looking for for dwell readings?

Steve
#4456

[DMCMW Dave]

Mine does the same - Rough idel and the occasional shimmy. Poor acceleration in the low RPMs and around 2200 it kicks in.

I have a really bad mix. Won't pass emissions and the duty cycle is all over the place and won't settle.

I replaced the ECU... that seemed to help for a bit but now I'm back to where I was before.

I only have 10,000 miles on the car so I don't think it's the lambda sensor.

I'm pretty confident I'm dealing with a vacuum leak but I'm not sure where.

So are duty cycle and dwell interchangeable? I was thinking about buying a meter that measures Duty but they are expensive. Harbor Freight (yeah I know) has one that measures dwell for $40. How do I hook it up to meassure and what am i looking for for dwell readings?

Steve
#4456

45 degrees dwell = 50% duty cycle. 90 degrees = 100%.

I've had bad luck with the Harbor freight digital meter. Impedance is too low or something. Never could get a good reading.

You can foul the O2 sensor with just a few minutes of running too rich. When that happens it runs open loop i.e. acts like the O2 sensor never warms up. You can tune the engine to run acceptably with a dead O2 sensor so that's probably not it.

I think I'd do an injector balance/flow test at this point. Then you really know what's going on.

[Grover]

Hmm, new symptom is a slight gas smell in the cabin after a long drive and the car is left to sit. I ran the car and opened up the fuel pump area and checked all the lines and the injectors in the engine bay and I don't see any fuel leaks. I am unfortunately out of state until Thursday, wish this happened while I was at home.

[Grover]

45 degrees dwell = 50% duty cycle. 90 degrees = 100%.

I've had bad luck with the Harbor freight digital meter. Impedance is too low or something. Never could get a good reading.

You can foul the O2 sensor with just a few minutes of running too rich. When that happens it runs open loop i.e. acts like the O2 sensor never warms up. You can tune the engine to run acceptably with a dead O2 sensor so that's probably not it.

I think I'd do an injector balance/flow test at this point. Then you really know what's going on.


I have replaced the O2 sensor a couple of times while trying to figure out the issue the first time. Sad to think I have to do it again. Last I checked the injectors in April they were all spraying a good pattern/volume.

[jmettee]

Steve, you probably can't get the car on a lift until you get home, but if you have a leak in the area of the fuel accumulator or the fuel filter, you may have a leak & fumes are making their way in from under the console & shift boot. This would explain poor mileage & fuel smell. If the leak is bad enough, I can see how the fuel injection pressure would be low & cause stumbling. However, I would think that leak would have to be significant to be causing those types of problems & you should easily be able to see fuel dripping/spraying from under the car.

If you can pull the plugs (all 6), they might tell some of the story (i.e. all black, then you are definitely running rich). Perhaps the Cold Start Valve is spraying all the time....try starting the car, unplugging the CSV, & taking a drive to see if that changes anything. While you're at it, check all the "easy" connections by unplugging them & plugging them & plugging them back in. These would be the sensors in the thermostat housing, on the distributor advance, on the frequency valve, & all the harnesses in the engine bay corner under the cover. Also, check the spark plug boots & make sure they're tightly on the plugs. Checking the distributor end is not really possible with K-Jet in the way.

[Grover]

Steve, you probably can't get the car on a lift until you get home, but if you have a leak in the area of the fuel accumulator or the fuel filter, you may have a leak & fumes are making their way in from under the console & shift boot. This would explain poor mileage & fuel smell. If the leak is bad enough, I can see how the fuel injection pressure would be low & cause stumbling. However, I would think that leak would have to be significant to be causing those types of problems & you should easily be able to see fuel dripping/spraying from under the car.

If you can pull the plugs (all 6), they might tell some of the story (i.e. all black, then you are definitely running rich). Perhaps the Cold Start Valve is spraying all the time....try starting the car, unplugging the CSV, & taking a drive to see if that changes anything. While you're at it, check all the "easy" connections by unplugging them & plugging them & plugging them back in. These would be the sensors in the thermostat housing, on the distributor advance, on the frequency valve, & all the harnesses in the engine bay corner under the cover. Also, check the spark plug boots & make sure they're tightly on the plugs. Checking the distributor end is not really possible with K-Jet in the way.

Justin,

I tried unplugging both the cold start injector and the pressure regulator, both made the car run worse. I will have to wait until I get home with the dwell meter to confirm the other ideas.

I will also check all of the connections. The systems did start right after it rained, but I have washed the car before with no problems. Maybe one of the spark plug boots did become loose. I will know when I have time to troubleshoot on Friday.

The accumulator hoses were all changed two years ago, though maybe they went bad already? I will have to wait until I can jack up the car. The fuel filter became loose once before, and that was fixed and tightened two years ago as well.

It is just so weird that it drove 100% perfect, then I shut it off, ate dinner, and on the next start up it has not been right since.

[Grover]

OK, here's an update


- The fuel smell is from a torn boot by the return line.

Onto the engine shake...

- The lamda system is working...I still get a dwell signal of 45-47, about 55-56 when full throttle switch is hit'

- Spark plugs look good

-Spark plug wires have spark

- cylinder compression is good

- fuel injector spray is good


Here is where the problem is. The engine is lumpy at idle because there seems to be a bit of a "miss" when running. After having it looked at by two other owners, they noticed that on one cylinder you can unplug the spark plug wire and there will be absolutely no change in the way the engine runs. The engine runs the same way whether it is plugged in or unplugged.

Does anyone have any ideas about this? I have checked all grounds, electrical connections and relays. I plan on pulling the motor this winter with some friends to do a rebuild, but I still want to figure out why it is suddenly running the way it is.

[Bitsyncmaster]

Have you done a compression check yet?

A miss firing cylinder will make the lambda system run lean since it will see that raw fuel in the exhaust.

[content22207]

Regarding lean fuel/air mixtures:

In 1971, engineers from the AC Delco division of General Motors scientifically proved that smaller spark plug gaps (less than .03") and lower secondary ignition voltages are more liable to misfire lean fuel/air mixtures than larger spark plug gaps and higher secondary ignition voltages. They published their findings as Society of Automotive Engineers Paper #720007 (February 1972). In the discussion that followed, engineers from Lucas, Ford, and Champion Spark Plugs concurred with these findings (not a single dissenting argument was presented). Not coincidentally, nearly every car manufacturer switched to higher secondary ignition voltages and larger plug gaps shortly thereafter, and never went back to the lower secondary ignition voltages or smaller plug gaps that were common prior to 1972.

Remember that historically, North American fuel/air mixtures were leaned throughout the 1970's for both fuel economy increases and emissions reductions. A Lambda equipped DeLorean is a perfect example of a car that was intended to run lean. Curiously, DeLorean ignition differs significantly from nearly all of its contemporaries. Other than the breakerless ECU, DeLorean ignition is actually closer to 1960's ignitions than 1981 ignitions, especially in terms of secondary voltage and spark plug gap.

Bill Robertson
#5939

[TTait]

To be clear then, what are you running and what makes you happy as far as coil and gap Bill?

[content22207]

That has already been discussed elsewhere on this forum.

All I'm saying is if Steve's fuel/air mixture is running lean, he could indeed be misfiring -- moreso than he might expect from his experiences with other vehicles.

Bill Robertson
#5939

[Grover]

The compression is good.

About the fuel/air mixture. Nothing was touched from when it was running perfectly, shut off for 5 hours and then started back up again. Unless there is something else other than the mixture screw that can adjust the fuel mixture that I am missing.

[Bitsyncmaster]

You did check that injector with the dead cylinder? And you are seeing spark on that plug? Then what I would do is replace the plug and wire. You could just swap those parts with another cylinder and see if the problem moves.

[jmettee]

Given that you located the "bad" cylinder, I would suspect a bad spark plug, bad distributor carp (could be corrosion on that pickup in the cap), or the plug wire fell off the distributor cap. I would suspect these simple causes rather than mixture since it happened all of a sudden & without tinkering on the engine.

[Ron]

If more information or facts are needed, let me know.

-Steve

Great advice given already!
If your KJet isn't missing, you can still do a reasonable test of the distributor end using a known good spare plug...

Does the miss act the same if you pull the plug wire at say 1400+ RPM STEADY?

[Grover]

You did check that injector with the dead cylinder? And you are seeing spark on that plug? Then what I would do is replace the plug and wire. You could just swap those parts with another cylinder and see if the problem moves.

Checked the cylinder compression, spark and fuel injector spray. All were normal. I'll try swapping a wire later. Even if the wire is producing spark it could be bad?

[Grover]

Given that you located the "bad" cylinder, I would suspect a bad spark plug, bad distributor carp (could be corrosion on that pickup in the cap), or the plug wire fell off the distributor cap. I would suspect these simple causes rather than mixture since it happened all of a sudden & without tinkering on the engine.

I'll check the connections on the cap when I get home this weekend. The wires and plugs and cap are about 1 1/2 years old.

Another idea was the thought of moisture within the cap. I have been driving it in more rain than usual this year.