[Unknown User]

> It's a '78, CA emission car, stock, minor
> tuning mods.

> About two weeks ago, it started running
> rough until warm-- ei., coughs and sputters
> until about 3,000 rpm them smooths out until
> I shift and the rpms drop again. This all
> started not long after I did an engine
> clean, and I expected corroded connectors on
> cold start valve or thermotime, etc. Went
> through and cleaned them all off, coated
> them with dielectric grease and still no
> change.

> One other thing that seems wierd to me. When
> I first crank it, it runs perfectly smooth
> at idle (900 rpm) for about 10 seconds
> before it starts it coughing and sputtering
> at 500 rpm. Then, once it starts to warm up
> (still cold on the gauge, but running
> smoother) the rpm will fluxuate and cycle
> between 500 and 900 until the car warms all
> the way up.

> Once it's warm, she runs and idles
> beautifully.

> Where do I start? How can I check the cold
> start valve if that could be the culprit?
> Any ideas or help would be appreciated.

> Thank you for your help!!!
Matthew, you said that you cleaned the engine. Could you have got moisture in the electronics, by steaming cleaning, or pressure washing it?
Check for moisture in the throttle position switch, on the side of the throttle body, I have seen those cause the idle to fluxuate. Joe

 

[Unknown User]

the cold start valve and thermotime swith really don't come into play in this situation. when the cold start valve works, it is only for a few seconds. the thermotime switch doesn't ground the cold start valve over a coolant temperature of about 60 degrees or so, usually.if the car is easy to start and runs well when first started, these parts should be fine. the best thing to find out is if the car is running rich or lean when the problem occurs. this makes the problem much easier to find. i would start by checking the cylinder head temp sensor. it should be between 2.1 and 2.9 kilaohms at 68 degrees. check it with the car hot and cold. the resistance should drop as the car warms up. if the ecu thinks the car is warm when it is really cold, it will shorten the injector pulse width, making the car run lean when cold. hope some of this helps.

 

[Unknown User]

Re: I'll keep trying

> Thanks for your responses. I'll keep trying.

> Would loosening the spring on the AFM cause
> it to run rough until warm?

it's possible, loosening the spring is supposed to make the car run richer at any given rpm or temperature. is your car running rich or lean when the problem occurs?

 

[Unknown User]

Re: Rich or Lean???

> Other than the smell of unburned gas or
> using an emissions probe, how can I tell?
> What is the spark plug supposed to look
> like? And if it's running rich, (this may be
> a stupid question) why would it only run
> poorly while cold?

you can look at a plug. it will be a wet black color if it running rich. the best way to tell is the smell. also if the car is running rich and you give the car a vacuum leak, the idle should pick up. i would think your car is running lean when cold. the fuel injection is very sensitive to the head temp sensor. it's possible that the car runs overly rich when cold and then goes back to normal when warmed up but it's more likely it's running lean when cold.

 

[Unknown User]

Re: Rich or Lean???

do a head temp sensor check. i would set your afm back to the stock position, at least until you fix the car.

 

[Unknown User]

Re: Rich or Lean???

you should do a sensor check on your fuel injection system. the cold start valve can stay on causing it to run rich, check it and the thermotime switch. check the throttle switch for a wide open position short, check the head temp sensor. also check the air temp sensor in the afm. also the fuel injectors can stick open. are there any other symptoms? does it run rich all the time? just when cold? when hot? is it hard to start when cold or hot?

-casper

 

[Unknown User]

Re: Fixed, I think + ???

> Well, I have a foot of new snow on the
> ground and it's comming down hard here in
> Utah. So I'm not going anywhere soon in the
> Z to see how it really runs, but... started
> it in the garage this morning and it idled
> great. Seemed to rev well while sitting
> which it never did before. So I'm pretty
> much convinced I had the AFM spring too
> loose. I might have to adjust it one more
> time, but so far I'm pretty encouraged.

> Thanks for your help.

> Another dumg question. For optimum
> performance, should this fuel enjection be
> running a little rich, right in the middle,
> or a little lean? Does the scenario change
> with rpm in the power band? And if I change
> to a performance cam, does this scenario
> change?

> Thanks in advance.

> Matt

usually right in the middle is the best for performance. if you run a little rich, then that's all your doing, the excess fuel doesn't get burned and contributes no extra power. your mileage will suffer. a little lean doesn't do any good either. right in the middle is where you want to be, but if you can't get it perfect, always run on the rich side. you won't notice the effects as much. if you change cams you may have to make some changes. the motor will be able to swallow more air so you may have to loosen the spring and make a co adjustment for peak performance.

 

[Unknown User]

Re: rich fix?

> okay even with the cold start valve
> disconnected it runs rich.
> it runs rich all the time, the exhaust
> smells of unburned gas, and when i
> disconnect the vacuum the idle picks up

> just recently i tookthe head off to change
> the head gasket because it was blown when i
> bought the car( a couple of months ago) so
> might this affect the fuel mix if something
> was set for the head before could the lack
> of coolant into the engine affect that? (the
> gasket was leaking coolant and a little bit
> of oil)

I have a 75 280z runs real rich (vacuum leak causes engine to rev way up). How much should I adjust the spring. It also has 3 sensors on the front of the thermostat housing. Which one is which. Thanks Sam 75 280z

 

[Unknown User]

Re: rich fix?

> okay even with the cold start valve
> disconnected it runs rich.
> it runs rich all the time, the exhaust
> smells of unburned gas, and when i
> disconnect the vacuum the idle picks up

> just recently i tookthe head off to change
> the head gasket because it was blown when i
> bought the car( a couple of months ago) so
> might this affect the fuel mix if something
> was set for the head before could the lack
> of coolant into the engine affect that? (the
> gasket was leaking coolant and a little bit
> of oil)

bear in mind the injector can still leak, even after disconnecting the power, if the valve is stuck open. do you have a california car with an oxygen sensor? if so burning oil and coolant can coat them with crud, and they won't work anymore. have you done a sensor check yet on the head temp sensor?