divil

I've been trying to track this down for a while...I have a nasty shuddering problem at very low throttle when the engine is cold (stock 89 951). The idle is stable but if I add just enough throttle to click the tps, the engine begins to stumble until I either add more throttle or back it off to idle.

If I unplug the tps, then I have the stumbling at idle. All of this only happens when cold, for a few minutes at most, and it's worse on cold days. I have replaced the DME temp sensor and O2 sensor recently. Other than this problem, the car drives ok and I get full boost.

Then I read that there is a temp sensor in the AFM...I checked it against the table on this site (http://www.the944.com/afm.htm) and it seems off. Last night it was around 11 degrees Celcius and the sensor showed me 1.18K. Going by that table it should have been between 4.4 and 6.8 though?

I took the AFM in and cracked it open and cleaned all the contacts. The wiper mechanism looks ok - I can see 2 lines in the track but can't see anything wrong with the output voltage as I turn the wiper, except that there is a few degrees of free movement at the start and end of the track that doesn't change the output - but it looks like it's designed that way.

This morning after I reinstalled the AFM, the temp sensor was showing a little over 3K, but it was 6C outside, so it should be at least as high as 4.4?

Am I reading all this correctly?

Here is a video of the symptom: when you see the shuddering, that is me gently pressing the throttle...


Thanks!

F18Rep

Sounds logical but next time you have the DME out, check your FQS...Bruce

divil

OK will check that.

I think I can put my suspicions about the afm temp sensor to the test by faking a higher resistance. I'll just have to wait until the car is cold again to do a valid test...

divil

Well this morning it was closer to being in spec, 4.2K and about 7C outside. I pulled the connector and jumped all the wires, except I put a 5K resistor between the temp sensor output and ground....no improvement at all.

944hal

I had something similar. Car would start and idle but would not take any throttle input when cold. OK when warmed up. I know the chip I was using was causing the car to run rich. I switched DME chips and the problem went away. Car starts easily when cold and takes throttle immediately. You didn't mention whether your car was chipped or if you had changed something recently.

divil

Yeah that sounds like the same thing! I believe the car is 100% stock. I have not actually looked at the DME chips but the PO said the car was stock, and I believed him. Mechanically, it's bone stock.

I've found a few old threads describing a similar problem but they all seem to end with no resolution

I will check the FQS and the DME chips and rule that out once and for all.

I've noticed that I get the stumbling any time that (a) the engine is cold and (b) the ISV is off. I think that is why adding a little throttle is significant, because the TPS switches the ISV off. But if I deactivate the ISV by jumping the diagnostic pins, or by unplugging the TPS, then I see the same misfiring - at idle. Why can the engine run at low revs with the ISV, but not without it? All the ISV does is add more metered air, right?

bordin34

I have/had a similar problem.
It used to be that I had a bad hesitation when cold where I had to floor it to get good throttle response. Otherwise it would stumble. Sometimes it would happen sometimes it wouldnt. The colder out it was the worse it was. Sometimes it lasted 10 minutes. It only happened when what would be the "choke" is on. I gave up fixing it.
Then in the summer I replaced the oxygen sensor. The problem is almost entirely gone. It will happen the first time I take off from a stop and thats it. The oxygen sensor shouldnt do anything since it isnt used while warming up but somehow it helped.
Prior to that I tested my temp sensor, afm, tps, and did a full tune up. I have no idea if I have a stock chip or not. My car is an 86 N/A.

Hollywood D

Mine does the same thing. Haven't been able to figure it out.

turbo944

Get some of that symptom myself...AFM tested good but didn't get a chance to test the temp sensor there yet. But in the process of all the work I did there's new plugs, wires, coil, TPS (which did test bad when hot), partially rebuilt throttle body and cleaned, cleaned the AFM, all new vacuum lines, cleaned idle stabilizer, new speed and ref sensors and cleaned grounds.

If it's warm outside I have no issues....it's only when cold and it just falls over on it's face a few times, almost as if there's a block to the airflow making it to the engine. Once the engine warms up it's great.

Wondering if somehow the AFM door is sticking but mine didn't seem to have any issues that I could find. Otherwise maybe a sensor somehow elsewhere?

I do have Guru Racing chips which have been in for quite some time and I never used to notice the issue years ago when they went in...this has developed since the installation of the chips in the last 3-4 years...occasionally at first and now more regularly.

Oh, and no oil or coolant use now either (well, after I fixed the oil pan gasket, etc. and did all new water lines, heater valve and radiator...it holds 10PSI for an hour with no change), so I find that hard to figure out.

divil

So I tested my AFM properly - with the engine running, and here's what I get:

cold idle (ISV on and hissing loudly): 1.2v
cold, just above idle (ISV off): 0.99 - this is when the misfiring happens. The voltage goes down!

warm idle (not sure about the ISV lol!): 0.6v
warm, just above idle: 0.6+ - engine revs up perfectly, but voltage drops to 0.5 when I release the throttle to idle, and a brief stumble occurs until the voltage climbs back up to 0.6v

So the problem is obviously linked to the AFM output...the question is whether or not the AFM is bad or is being thrown off by a vacuum leak, or what. But I think I can rule out a ton of stuff now (and also BTW, I checked my DME and I have the stock chip, and FQS set to the stock position)

Any insight into this behavior? And can anyone explain why there is only 1/2 the quantity of air coming in at warm idle compared to cold idle? Or is that an indication of a bad AFM?

MarkX0

This is the very reason i want to do an AFM to a MAF conversion... like Rogue Tunings (I want the kit so bad, hopefully i can order it soon). The stumbling and hesitation is annoying and the feeling of the car jumping when it finally catches drives me nuts. The symptoms usually come from the worn out track that the wiper makes contact with that's why there are 2 lines on the track. I've been through a couple of AFM and after opening one up and moving the wiper to a bit to fresh track, as described in FRWilks website, the hesitation and stumbling was gone even in the cold. I was afraid to mess something up when I moved the wipers, but since I had spares I risked it and it worked out pretty well.
This is FRWilks website describing the AFM and the youtube link is a good explanation.
http://www.the944.com/afm.htm
https://www.youtube.com/watch?v=-11J8XawQtc

divil

Well I am not sure the problem is the afm in my case, I think it's a vacuum leak. I haven't been able to find it yet but I am going to do a thorough test at the weekend. I can't explain why the problem goes away as the engine warms but I believe it's either an expanding pipe sealing up the leak, or it's connected with the ISV, which gets used less as the engine warms.

But I know what you mean all the same. It's would be hard to have confidence in the afm if you have been through a few, and the cost of a new one is hard to justify when there are good MAF kits out there.