konakat

After a couple close calls in the past my 944 finally forced me to call for a ride last night.

I have found that if I drive the car much more than 20 minutes the engine starts surging wildly and ends up stalling. I've always managed to limp home in the past, but this time I had to let the car cool for several hours before I was able to drive it again. It always manages to start up after it stalls, but will only run for 10-20 seconds before getting all wonky again. It isn't a simple overheating issue, when the temp gauge gets to about 3/4 the fans kick on and bring it back down to around 1/2. I'm thinking that there is a sensor that stops working correctly once the engine is really thoroughly heat-soaked. Has anyone seen this before? It is difficult to diagnose since it requires running the engine for quite a while before it happens.

odonnell

Weird. AFAIK the only temp sensor that affects engine performance is the DME temp sensor. It adjusts the EFI for cold/hot performance. It sits in the block right under the area where the J-boot mates up to the throttle body.

The fans shouldn't be kicking on that late... I recommend venting the system and cleaning grounds. Particularly, the grounds on the driver's side of the radiator, right behind the headlight, as well as the grounds over the bellhousing[/url].

Air in the system allows superheated water (steam) to build up, which registers at a much higher temperature than the coolant (latent heat). This might cause the DME temp sensor to get wacky readings and corrupt EFI adjustment. Just a theory

John_AZ

Check the fuel delivery.
Fuel Pump pressure per WSM
FPR=fuel pressure regulator with pressure guage.
FPD=fuel pressure damper---pull the vac lines and see if any drips of gas present.

See pages in WSM SOHC pages 24-4 to 24.7

You can get a full set of pressure gauges or I just use an old fuel rail cap threaded for a cheap pressure gauge.



The O2 sensor adjusts the ISV idle via the DME.
Unplug the O2 sensor and see if it gets better. Buy Bosch OEM if needed and not an aftermarket.

GL
J_AZ

konakat

I don't think its an actual temp sensor since the engine runs fine at full temperature for 15 minutes without issue. I read this morning that the DME relay is often temperature sensitive and will stop working once it gets warm, so hopefully that's it. I'm a bit skeptical since I would think that would just cause the car to stop dead without the surging and also prevent starting. I suppose it could also be a failing position sensor that has become heat sensitive. I'm certainly open to other ideas.

konakat

Hmm. I just recently reconnected my O2 sensor, but I have seen this problem before I did that. It never stranded me before, though, so maybe the sensor has made it worse.

Van

I bet it's either a DME problem or a fuel delivery problem...

An O2 sensor or temp sensor won't make it that undrivable.

John_AZ

....and I should never assume that the OP has tried his spare DME fuel pump relay or the Jumper per Clarks before I advise to check the fuel pressure.

http://www.clarks-garage.com/shop-manual/fuel-05.htm

J_AZ

konakat

Is there an easy way to induce a DME relay failure? Maybe a hair dryer lightly blowing on the relays while the car runs?

gtroth

Huh? No idle stablization on that car (no ISV). And for cars that have it (16V cars, not sure about turbos) the idle control loop doesn't look at the O2 sensor, just RPM, TPS, and temperature. When its time to idle, ISV is dynamically adjusted by the DME to hold rpm @ 850 warm, 1050 cold.

The 8V cars' idle control loop (with ICV) look at TPS and temperature.
iCV is open when cold to raise the rpm some, closed when warm. Idle rpms are set by the idle screw. When you are idling normally, what is your rpm?

Surging idle would steer me to start with TPS. Theory is that the throttle is closed but the TPS is not telling the DME it's time to idle. So car starts to stall, and DME dumps more fuel to prevent it. Idle bounces. So check the switch.

There's another way for the TPS function to fail with a good TPS, btw. After a throttle rebuild, my 'S' idle would occasionally bounce like that when it heated up which turned out to be the throttle plate rubbing the throttle body and preventing the TPS from actuating. Not saying a sticky throttle is your issue necessarily, but I think it illuminates what's going on a little. Might as well rule it out- when idle is bouncing, grab the throttle cam and try to close the throttle a little more.

At times like this measuring everything you can is a good way to go, and can be very satisfying to know what is good and a terrific way to learn the car. So measurements of fuel pressure and leakdown, temperature sensor resistance, AFM resistances, speed/ref if you can, are worthwhile in my view.

Good luck!

ZR8ED

I'll bet it is fuel related. A bad fuel dampner. If it goes bad, the fuel pressure spikes, and MAYBE you can still drive it under load, but it will bog and stall when you try to idle it. Turn off the car, and the fuel pressure will eventually drop back down. Once it gets low enough, you can restart the car. Mine partially failed and I was able to limp it home where it completely died. I could start the car for a few seconds, then the fuel pressure would spike 100psi and the car would stall.

Goodluck and report back!

John_AZ

Thanks, my mistake on the OP 1984 944--NO ISV .

My basic info on the O2 sensor explained in laymans terms:

http://forums.pelicanparts.com/1675972-post9.html

J_AZ

gtroth

Thanks, John, handy link.

The air/fuel stoiciometry control loop does start once warmed up so is synchronous with the bad behavior. The loop should be running independently from the idle control loop, but I bet there is some interaction (ie idle control while in air/fuel-closed-loop-mode might behave a little differently than when in open loop mode under certain conditions). So agree we shouldn't take O2 off the table yet.

konakat

Any suggestions on an affordable fuel pressure gauge that will fit the Bosch system? Is my best bet to create an adapter for the Harbor Freight gauge?

divil

Here you go http://www.arnnworx.com/catalog/inde...roducts_id=191

That one will make a perfect seal on the factory fuel rail without needing sealing tape or anything like that. I tried making an adapter form a spare fuel rail cap for the HF the one, following the procedure on clark's garage, but it leaked all over the place.

That said, I am at a loss to make any sense of the reading on my Arnnworx one. As you can see he used a 40 bar gauge for measuring 2-3 bar pressure

konakat

I can rule out the TPS at least. Keeping the throttle down doesn't help the surging, and I actually have the throttle adjusted so that the idle side of the TPS doesn't activate. I was having the same problem before I made this change so at least I can rule that out.