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Discussion Starter · #1 ·
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!!!
 

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Discussion Starter · #3 ·
> 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

Yea, it did get moisture in some of the connections. The symptoms were different-- wouldn't rev past 2800. But I already cleaned and dried those out. And it was after that happened that the cold problem started.

Matt
 

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Discussion Starter · #6 ·
Re: I'll keep trying

> 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.

Thanks for your responses. I'll keep trying.

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

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Discussion Starter · #8 ·
Re: Rich or Lean???

> 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?

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?
 

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Discussion Starter · #10 ·
Re: Rich or Lean???

> 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.

OK,
I'm going to check to see if I can see any signs of running too rich. If I do...? I guess I'll try tightening up the AFM a couple of notches and see if that does it. If there's no sign of rich, I guess we can assume it's running lean in the cold start-up period. If that's the case, what should I look for next to correct the problem?

Thanks
 

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Discussion Starter · #12 ·
Re: AFM

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

Casper,

I fiddled with the AFM while it was still cold and running rough. It seemed as though I was able to find a position that smoothed everything out. I think this may have been the problem. I'll try it in the morning when cold and let you know.

Thanks for all your help.

Regards,

Matt
 

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Discussion Starter · #15 ·
Re: Fixed, I think + ???

> 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

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
 
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