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 Post subject: Re: MK1 erratic rpm VAF?
PostPosted: Fri Mar 15, 2013 10:56 am 
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Joined: Sun May 06, 2012 3:07 pm
Posts: 60
Location: Elkridge, Maryland
Im pretty sure that its either the spring in the door to the VAF or the potentiometer inside is shot. I ohmed it as it says to do in the MK1 supplement. with the door fully closed its reading over 500 ohms. spec is 20-400 ohms with door closed and 20-1000 with door open. A vacuum problem would give me drivability issues but it wouldnt give me a rediculous rpm reading while the car is trying to idle and then run completely fine on steady throttle anywhere after 2500 rpm. Vacuum readings are very strong (22in at idle and slow rev up) as Ive said before, and there are no fluctuations indicating a burnt or cracked valve. All the vacuum lines are very good quality still and ive even gone as far as to block off individual lines just to make sure it wasnt them. There is no vaccum leak.

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1988 Chevy Sprint Turbo
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 Post subject: Re: MK1 erratic rpm VAF?
PostPosted: Fri Mar 15, 2013 11:18 am 
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Joined: Tue Feb 13, 2007 10:28 pm
Posts: 1170
Location: So Cal, USA
Quote:
A vacuum problem would give me drivability issues but it wouldnt give me a rediculous rpm reading while the car is trying to idle and then run completely fine on steady throttle anywhere after 2500 rpm

A leak anywhere in the system will give you a bad, stumbling idle, like the car wants to die.
Higher revv's will overcome this and the engine will sound more steady.
The system is meant to be sealed, any crankcase vent leaks or weak spots in the valve
cover or intake will cause a bad idle. It is the opposite of the normally aspirated system.

Try creating a break in the system. While the car is idling, pull out the oil dipstick...
if the engine runs rougher and/or dies this is normal, however if there is no change
then it will confirm you have a leak somewhere in the crankcase vent system.


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 Post subject: Re: MK1 erratic rpm VAF?
PostPosted: Fri Mar 15, 2013 12:49 pm 
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Joined: Sun May 06, 2012 3:07 pm
Posts: 60
Location: Elkridge, Maryland
Well, Ive pulled the dipstick while the car was running back when it ran great and there never was much of a difference. Oil cap stalls it every time before and still now. Pull the pcv tubes or anything and it stalls now. But before it would just run rough. I just checked the distributor cap and rotor. All ok. Put an inline spark tester on and spark is steady and strong even when car is misfiring. Next step Im going to buy a injector noid light and see if the ecm is cutting the fuel. Does anybody think a bad coil signal could be causing this erratic tachometer reading?

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1988 Chevy Sprint Turbo
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 Post subject: Re: MK1 erratic rpm VAF?
PostPosted: Fri Mar 15, 2013 4:30 pm 
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Joined: Thu Aug 16, 2012 8:53 am
Posts: 100
Location: Northwest
I am not sure if someone mentioned this, and it might sound dumb, but check your grounds. The fuse block under the dash definitely requires a solid ground in particular.


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 Post subject: Re: MK1 erratic rpm VAF?
PostPosted: Sat Mar 16, 2013 10:53 am 
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Joined: Sun May 06, 2012 3:07 pm
Posts: 60
Location: Elkridge, Maryland
Yup re did all the grounds. Grounds can be crossed off the potential problems list.

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1988 Chevy Sprint Turbo
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 Post subject: Re: MK1 erratic rpm VAF?
PostPosted: Sat Mar 16, 2013 11:21 am 
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Joined: Sun May 06, 2012 3:07 pm
Posts: 60
Location: Elkridge, Maryland
JamalSpelling wrote:
Quote:
A vacuum problem would give me drivability issues but it wouldnt give me a rediculous rpm reading while the car is trying to idle and then run completely fine on steady throttle anywhere after 2500 rpm

A leak anywhere in the system will give you a bad, stumbling idle, like the car wants to die.
Higher revv's will overcome this and the engine will sound more steady.
The system is meant to be sealed, any crankcase vent leaks or weak spots in the valve
cover or intake will cause a bad idle. It is the opposite of the normally aspirated system.
.


Went out last night to show my buddy how the car was acting. With a nice bright flashlight you could see some smoke come out the side of the valve cover. I was never able to see the smoke a day :lol: Going to take care of that leak and see if it doesnt fix her up.

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