944 engine hunting

[big al]

hi

i have a 1986 944 when i am under full acceleration everything is fine also at idle all is well but at constant revs trying to hold the same speed i can only describe it as if the engine is hunting, to me it cant make up its mind on how much air/fuel mix to give the engine, my thought are

1. air flow meter
2. fuel regulator going bad

don't know if this is related but my DME RELAY failed yesterday, i thought thats lucky i have a spare one, then oops its 20 miles away at home, not much use there i know but at least i remembered to get one also i have already replaced the fuel filter,spark plugs,rotor arm,distributer cap,air filter and retracted the air flow meter as per Clark's garage, most of this was done to solve a misfire problem, that has solved one fault but i am left with what i descried as a hunting engine


cheers big al

 

[Guest]

ORIGINAL: big al

hi

i have a 1986 944 when i am under full acceleration everything is fine also at idle all is well but at constant revs trying to hold the same speed i can only describe it as if the engine is hunting, to me it cant make up its mind on how much air/fuel mix to give the engine, my thought are

1. air flow meter
2. fuel regulator going bad

don't know if this is related but my DME RELAY failed yesterday, i thought thats lucky i have a spare one, then oops its 20 miles away at home, not much use there i know but at least i remembered to get one also i have already replaced the fuel filter,spark plugs,rotor arm,distributer cap,air filter and retracted the air flow meter as per Clark's garage, most of this was done to solve a misfire problem, that has solved one fault but i am left with what i descried as a hunting engine


cheers big al

Big Al,

I am sure Jon Mitchell or one of the 944 NA guys will be around soon and give you the benefit of their experience but in my experience hunting is usually caused by either a leak on the intake/vacuum system, throttle position sensor or air metering device i.e Air Flow meter measuring incorrectly.
The old way to locate an air leak involved a can of Bradex Easy Start with the tube attached and give small bursts around specific areas of the engine i.e inlet manifold gaskets throttle spindles etc and see if the revs increase. If the revs do increase you have an air leak. As I recall Clarks garage give a test procedure for the Air Flow Meter and TPS too. Its worh going through the motions it may be something simple.

 

[944Turbo]

I would suspect the idle stabiliser valve which has a habit of getting gummed up and is supposed to stabilise the idle. There is a procedure that bypasses it for setting the idle speed - I would try this and see if it makes any difference. Clarkes garage should have the procedure, and there has been discussion here before on the ISV.
Tony

 

[Indi9xx]

If its ok at tickover and ok at full throttle and only happens in cruise situations, I would suspect its the airflow meter being out of calibration.

We see the same thing quite often, we put the airflow meter on a flowbench and re-set the spring tension, overhaul the resister track and check over the units before returning them.

If your car was doing it at idle, I would go along with Tommo and the vacuum leak, including his recommendation with easystart or wd40 or with Tony's ISV diagnosis, or possibly a bad fuel pressure reg, or even poorly set mixture by the AFM bypass screw.

My money would be on a bad AFM... if you decide not to have yours repaired and get a used one.. make sure its from a pre 87 car.

 

[big al]

hi

cheers for the replies guys

i only get the problem in cruise situations, when at idle it ticks over fine also on full throttle it picks up and runs fine, so as has been suggested i am inclined to think its the air flow meter i have had a look of the AFM and the plastic cover has already been removed but i opened it up and had a look any way the groves are very bad i have modified as per Clark's garage so it runs on a new track but i suspect this has been done some time earlier in its life so if this does not work i will contact Porsche apart for a replacement


big al

 

[big al]

Indi9xx i would have thought the AFM would have been the same for all the 944 n/a cars, if not whats the difference? is it possible to get these repaired from Bosch or would a used one be more cost saving


the Bosch part number on mine is 0 280 202 064


big al

 

[Diver944]

ORIGINAL: big al

is it possible to get these repaired from Bosch or would a used one be more cost saving

I would check the cost of a used one (with potentially worn tracks) and then give Jon a call direct to check the cost of reconditioning yours. I had mine done as part of my engine rebuild, as well as the throttle body re-sealed and it is good value for money compared with a jerky and/or hunting engine.

 

[big al]

ORIGINAL: Diver944


I would check the cost of a used one (with potentially worn tracks) and then give Jon a call direct to check the cost of reconditioning yours. I had mine done as part of my engine rebuild, as well as the throttle body re-sealed and it is good value for money compared with a jerky and/or hunting engine.

the only thing is i am in Aberdeen so i would have to post it down to Jon for repair, that would be alright as long as he does not need the car to test it on again, porscheapart just quoted £80 plus carriage plus vat, i am looking at one from a breaker in th USA same part number as mine £25 plus £15 air mail thats a good price i think, he promises it has not been opened so even if it is worn i should be able to move the tracking pins on to a fresh part so they get good contacts again, i think at that price i might just go for the one in USA

 

[Indi9xx]

In 1987 Bosch changed the voltage to airflow curve of the output from the AFM at the same time as they changed the DME to match.

For the 2.5 Lux there are 3 important differences in DME and 2 important changes in AFM...The AFM from 82 to 87 is the same, and then from 87 onwards for the lux is the same. So if your car is from 82 -87 then any airflow meter from an 82 to 87 car will do you.. just make sure the breaker also knows this and will not send you an 88 one.

 

[big al]

ORIGINAL: Indi9xx

In 1987 Bosch changed the voltage to airflow curve of the output from the AFM at the same time as they changed the DME to match.

For the 2.5 Lux there are 3 important differences in DME and 2 important changes in AFM...The AFM from 82 to 87 is the same, and then from 87 onwards for the lux is the same. So if your car is from 82 -87 then any airflow meter from an 82 to 87 car will do you.. just make sure the breaker also knows this and will not send you an 88 one.

thanks for the information on the changes to the AFM across the years its always good to know as i guess others out there like me included never knew there was any difference and just assumed they were all the same

i have given the breaker the Bosch part number thats on mine and he has confirmed that he has one the same sitting on a shelf ready for posting so hopefully in a couple of weeks i will have it fitted and running again


once again thank you to every one that has help me

cheers guys big al

 

[Guest]

Hi Big Al,

I know it sounds tempting to get the secondhand one from the US but if Jon can recon yours for that kind of money I would go with it. He will not need your car as it is all based on resistance from memory.
What happens if the one from the states develops the same problems? Yes you can send it back and then get involved in long dialogues regarding refunds. Believe me I have been there, I regularly buy kit from the US and have had a few issues that you usually end up writing off as it is more hassle to pursue.
I am sure Jon would at least warrant his work so in the unlikely event there was a problem he would see you right.
You pay your money and take your pick!!
good luck

Tom

 

[Guest]

By the way Big Al,

WATCH OUT .........................

Hunting is illegal!!!!!!!!![]

 

[big al]

ORIGINAL: tommo951

Hi Big Al,

I know it sounds tempting to get the secondhand one from the US but if Jon can recon yours for that kind of money I would go with it. He will not need your car as it is all based on resistance from memory.
What happens if the one from the states develops the same problems? Yes you can send it back and then get involved in long dialogues regarding refunds. Believe me I have been there, I regularly buy kit from the US and have had a few issues that you usually end up writing off as it is more hassle to pursue.
I am sure Jon would at least warrant his work so in the unlikely event there was a problem he would see you right.
You pay your money and take your pick!!
good luck

Tom

i know what your saying and if it were possible to take the car to Jon i would but its a 1137 mile round trip for me from Aberdeen so as you can see its not really a option also i would have to hire a car trailer on top of that as i would not trust the 944 to get me there in its current state, by the time i add that all up including a hotel for a couple of nights i would be better of getting a new one from Porsche

its my fault for living to far north

i have to thank Jon for his help in tracking down the fault on my car

big al

 

[Guest]

But Jon wouldn't need your car, just your airflow meter[8|]

 

[big al]

oops sorry misread you reply to much Christmas spirit, my AFM has already been opened and been retracted at some point in its life and has very bad tracks on it, it still may be repairable i don't know as i am no expert, i think i will see what happens with this one from USA if it turns out to be no better i will post that one down to Jon for repair

 

[AndrewS]

Hi Al,

You should be able to purchase a new OE Bosch Air Flow Meter for about £180 (you'll need to check suppliers for an exact price).

You can check the AFM by measuring it's output voltage under the conditions you describe. This is a simple test that you can conduct with a voltmeter. Also check the AFM connector - this can contaminate with damp and moisture.

Under idle conditions, the revs are maintained by the ISV (Idle Stabilisation Valve) and under full throttle conditions, only a 2D map is used (based on RPM increments), hence I would agree with Jon's diagnosis that it is likely to be the AFM.

Regards,
Andrew