'86 944T - Runs at idle speed only when cold?

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

T o d d P a t t i s t - 16 Aug 2006 16:35 GMT

I've got a 1986 944 turbo. I love it, and it's in great
shape, however, it's picked up a problem I'm not familiar
with. In the morning, I start it and it seems to run great.
I back out OK and turn down the downhill driveway and
everything seems OK, but as I accelerate out on the road,
the engine RPMs suddenly drop to idle speed, as though I'd
pulled my foot off the gas, or the electrical system had
killed the spark, but the engine doesn't die, it doesn't
even seem like it will die, it just drops back to idle
speed. It did this twice as a hiccup, like someone quickly
cut the engine spark, then turned it on, but this morning it
was serious. If I gave it gas, the RPMs would rise to
perhaps 2200 max, then drop back to idle, then try to rise
again, then drop back. It did this 20- 30 times or more.
Nothing I did would give me higher RPMs, then suddenly, I
felt it begin to fire at higher RPMs and it's working
perfectly again.

As soon as it warms up and runs correctly, it runs fine from
then on. I've seen this now a total of 3 times in two
weeks, once in the morning and once leaving work - just as a
single or perhaps double hiccup, then this morning's serious
event.

Any suggestions on what this is?

Signature

The natural function of the wing is to soar upwards and carry that which is heavy up to the place where dwells the race of gods. More than any other thing that pertains to the body it partakes of the nature of the divine.

Plato, 'Phaedrus.'

Reply to this Message

darthpup - 17 Aug 2006 01:03 GMT

You need to clean the air flow meter valve in the intake plenum with
some gumout.

Reply to this Message

Bill - 17 Aug 2006 06:50 GMT

it could be as darthpup says - there is a motor controlled valve that allows
air to bypass the throttle plate and they do get sticky - and the valve
design changed between 85.5 and 87 (same exterior, different interior) - the
older design will wear out the motor commutator, the newer design doesn't
have a commutator to wear out.

But, I don't think that's likely to be the problem, that valve stabilizes
the idle.

There is an air flow meter right next to the air cleaner - it can get
sticky, but again, I've never seen this - what I have seen is on relatively
high mileage cars, the carbon track gets worn out and then you get a major
"dead band" effect when the airflow passes through the region where the
trackis bad - there are articles on hte web on how to fix this - I tried the
repositioning method and it was ineffective - a replacement sensor worked.
If you suspect the sensor, you can put a ohm meter on it and push the vane
with a pencil and see if resistance changes smoothly. Refer to the service
manual for some more details. there is a hot wire MAF replacement built for
the turbo cars that eliminates this part (and the wear problem) but it isn't
cheap

> You need to clean the air flow meter valve in the intake plenum with
> some gumout.

Signature

Posted via a free Usenet account from http://www.teranews.com

Reply to this Message

T o d d P a t t i s t - 18 Aug 2006 14:44 GMT

>it could be as darthpup says - there is a motor controlled valve that allows
>air to bypass the throttle plate and they do get sticky - and the valve
[quoted text clipped - 4 lines]
>But, I don't think that's likely to be the problem, that valve stabilizes
>the idle.

Hmmmm. A stable idle is not the problem. The idle seems
perfect. The problem happened again this morning. Idled
great, then wouldn't come up above about 2000 RPM and
dropped down to about 1000 every time I tried to get it
above 2000. It took about 2 minutes to warm up, then
hiccupped once very briefly, about 1 minute later, then ran
great for the commute.

OK, just to make sure I understand - there's a bypass valve
for air that could be sticky, or could have a worn
commutator. I can check this, but symptoms may not point to
that. - that's one possibility

>There is an air flow meter right next to the air cleaner - it can get
>sticky, but again, I've never seen this - what I have seen is on relatively
>high mileage cars, the carbon track gets worn out and then you get a major
>"dead band" effect when the airflow passes through the region where the
>trackis bad

And a second more likely possibility is an "air flow meter"
which I take to be a sensor telling it something about how
much air is going into the engine. Presumably it has some
sort of sensor vane that moves that could be sticky, but
more likely, the vane moves an electrical wiper on a carbon
track, and the track could be wearing out. The car is high
mileage (who's got an '86 that isn't?)

>- there are articles on hte web on how to fix this - I tried the
>repositioning method and it was ineffective - a replacement sensor worked.
>If you suspect the sensor, you can put a ohm meter on it and push the vane
>with a pencil and see if resistance changes smoothly.

This sounds like it to me. I'll need to do this when it's
cold, but it kind of matches what I'm experiencing. It
feels like the computer is shutting it down, not like an
actual airflow restriction. I'll look for the stuff on the
web.

> Refer to the service
>manual for some more details. there is a hot wire MAF replacement built for
>the turbo cars that eliminates this part (and the wear problem) but it isn't
>cheap

Thanks a lot for your help.

Signature

Reply to this Message

T o d d P a t t i s t - 18 Aug 2006 15:38 GMT

>>There is an air flow meter right next to the air cleaner - it can get
>>sticky, but again, I've never seen this - what I have seen is on relatively
[quoted text clipped - 4 lines]
>>If you suspect the sensor, you can put a ohm meter on it and push the vane
>>with a pencil and see if resistance changes smoothly.

Just a follow-up here - with your help, I found some great
info on the web about the air flow meter and the wear on the
sensor. I'll test and pull the air flow meter this
weekend. Thanks again.

Signature

T o d d P a t t i s t
(Remove DONTSPAMME from address to email reply.)

Reply to this Message

T o d d P a t t i s t - 21 Aug 2006 14:10 GMT

> I'll test and pull the air flow meter this
>weekend.

Here's an update for anyone interested in my 944T problem.
I pulled the AFM and did the sensor wiper repositioning
thing. I also cleaned the sensor vane. It operated
smoothly before I did this, and after installation, I waited
until it was cold on Sunday morning and .... the problem was
still there.

I carefully studied the symptoms:
When cold, the engine ran fine up to about 1500 RPM if I
slowly gave it gas, then it died and dropped back to idle.
If I pressed the gas harder and quicker, it would rise more
quickly and overshoot to about 2000 RPM.

Studying the manual on the tests for the DME, and reading
about the "shudder" problem for the 944 NA I realized that
in the trailing throttle condition (throttle closed, RPMs
high) the DME shuts off gas. The RPM threshold is...1600
RPM. This matched my symptoms. A quick test was to pull
off the throttle position sensor plug and short pins 4 and 6
in the plug leading to the DME, then start engine. Sure
enough, this produced symptoms that matched mine (by this
time the engine ran fine as it was warm) I verified this by
pulling off the DME plug and measuring there (pin 2 to
ground) that the throttle closed microswitch across pins 4/6
of the throttle position sensor switch was flaky.

Apparently, the switch was broken and was staying closed,
telling the DME that I'd removed my foot from the
accelerator, so the DME in an attempt to save fuel was
shutting off fuel flow above 1600 RPM. I pulled the switch
and tested, it, and it's bad. New one is on order.

Signature

T o d d P a t t i s t
(Remove DONTSPAMME from address to email reply.)

Reply to this Message

T o d d P a t t i s t - 11 Sep 2006 15:19 GMT

Here's a final update on this. I replaced the throttle
position sensor. For the 944T it has both a position sensor
(a potentiometer measuring throttle plate deflection angle)
and a switch that closes when the throttle is fully released
(opens when throttle is turned by one degree). The switch
part was bad and it had failed in a funny way - it stayed
closed some of the time. I think the internal microswitch
spring was broken that pushed it to the open position as the
throttle was pressed. The unit is sealed, but I was able to
pry it open and replace the switch with a tiny microswitch
that I had available. Most people couldn't do that without
hunting up a very tiny suitable switch, but I had dozens to
choose from. Not that it did me any good, as my wife had
already ordered the replacement - $88 aftermarket ($150
from dealer).

It works perfectly now. It is tricky, but you can remove
and replace this sensor without removing anything except the
air filter. The lower screw of the two that hold it on is
tough to get to.

>> I'll test and pull the air flow meter this
>>weekend.
[quoted text clipped - 30 lines]
>shutting off fuel flow above 1600 RPM. I pulled the switch
>and tested, it, and it's bad. New one is on order.

Signature

T o d d P a t t i s t
(Remove DONTSPAMME from address to email reply.)

Reply to this Message