Question about Idle

[shadowfax101]

Having trouble with a rough idle when warm. I say rough, I would almost describe it as a frequent jerk in the engine. It does not do it while warming up and idles very smooth when first started, but shows up after it has been running for a few minutes and after driving and stopping at a red light for example. When driving it runs fine and smooth with plenty of power(or what I would expect for these engines). It starts right away when cold or hot.

I have had the idle motor checked out and cleaned. I have installed new plugs, plug wires, cap, and rotor. I replaced the FV and the idle thermistor. I had a new O2 sensor about 10 months ago. I have replaced all the relays and fuses with the recommend upgrade from DMCH. I have installed all new vacuum hoses and replaced the little vacuum junction under the air intake. I installed a new micro switch and the vacuum solenoid for the vacuum advance .

I have not messed with the warm up regulator/CPR and I have not messed with fuel mixture. So what should I chase down next.

Thanks

[Ron]

I would suggest you check your CO. It's very easy to do with even the cheapest dwell meter you can find (analog not digital) and the workshop manual... All you have to remember is to use the 4 cylinder scale (or double 8 cyl) instead of 6 cyl.
Next, fill in all of your info in you profile...there may be some one near you to take a look...we live for it!

[Nicholas R]

How are your injectors? Is your ignition coil old or new? Has your fuel mixture ever been changed?

[DMC5180]

Sounds like you may have excessive idle hunt or it could be normal operation if your not familiar with the quirks of the delorean. The only time I get slight jerking is when transitioning in and out of idle circuit mode usually decelerating rolling up to a stop (in gear w/clutch out)

It's normal for the engine to run smooth at first start up when COLD after 3 minutes the lambda system kicks in and and idle starts to oscillate a bit. These cars don't run smooth and steady like a modern day car with EFI.

[shadowfax101]

i have reason to think that the injectors were a bit gummed up. I have been running a gas additive to try to clean them up. I have not pulled them to check, just the fuel lines and noticed that some of the bolts on top of the injectors has some old gas residue. The coil is the original one to the car. And I know that I have never had/or checked the fuel mixture.

[DMC5180]

What's the history of this car? Usage, ownership etc.

[shadowfax101]

I know very little about its history. It came from near Chicago and from looking at the names on the mailing labels from DMCH that were in the car it had about 3 owners before me that I can tell. The guy before me had it about a year and did not drive it much. I could tell it has mostly sat up for a period of time. I have had it for about a year and have been replacing most things on the car.

[DMC5180]

OK, That gives us a general idea. The more info you can give us since you've had the car the better. Running condition when you got it versus where it's at today would be good.

Welcome to the forum. :biggrin: Have you been following this forum quietly for the past year or did you just find us recently?


Please tell us your first name please. We don't bite.

[shadowfax101]

I have been reading and absorbing the info here for about a year. Most of the things I have done are ideas from here.

When I got the car, the idle was rough and seeking. I had the O2 sensor changed and it did have a sticking Idle control motor that I sent to John Hervey to have cleaned. The idle has been getting better all this time, I have been working on it with early issues being trouble with RPMs not right. A new micro switch and a solenoid for the vacuum advance fixed that.

The rough idle I have( or jerk) also corresponds to slight boom sound in the exhaust and, my FV buzzes while at idle continuously. Is that normal operation?

I do think new injectors and a new coil are on the list.

[shadowfax101]

oh...first name is John.

[DMC5180]

I have been reading and absorbing the info here for about a year. Most of the things I have done are ideas from here.

When I got the car, the idle was rough and seeking. I had the O2 sensor changed and it did have a sticking Idle control motor that I sent to John Hervey to have cleaned. The idle has been getting better all this time, I have been working on it with early issues being trouble with RPMs not right. A new micro switch and a solenoid for the vacuum advance fixed that.

The rough idle I have( or jerk) also corresponds to slight boom sound in the exhaust and, my FV buzzes while at idle continuously. Is that normal operation?

I do think new injectors and a new coil are on the list.


Continuous FV buzzing, is normal. It's the frequency rate that changes. Initially when Cold its a fixed rate. Then when the O2 sensor gets hot enough The Lambda computer starts varying the rate in an effort to maintain the lambda (stoichiometric) ratio.



....

[Nicholas R]

oh...first name is John.

Welcome John! :wrenchin::wave::cheers1::hihi::cheers::rock_on::c ool:

[David T]

To get a smooth idle each cylinder must work equally. If one has a dirty injector, low compression, misadjusted valve, etc, it will have a "bump" when it fires that cylinder. Hunting is normal but it should be smooth and consistent. On one car that had a bad idle the gaps on the reluctor inside the distributor were not all equal causing a bad idle. The closer you can get all of the cylinders adjusted to each other the better and smoother it will idle. Start with a compression test. If they are not all within 10% you will never get it to idle well no matter what you try to do.
David Teitelbaum