I've put about 400 miles on my 740 since buying it with a blown head gasket cause by a big overheat. I've done the HG, timing belt, tensioner, drive belts, vacuum hoses, air filter, wastegate hose, oil change, coolant expansion tank replacement. The car runs great on the road, but I'm having a lingering issue....when I start the car when hot, the idle stumbles and the car will occasionally die. I do not touch the gas at all. When I give the car gas, it will start and settle into a rough idle, but otherwise drives fine. Along with this problem is a very noisy fuel pump but it was noisy when I bought the car so might be unrelated.

What I've already done:
1. Replaced vacuum hoses.
2. Checked the FPR -- it seems ok as it pumps lots of gas from the return line at idle
3. Cleaned the idle control valve.

Cold start is great and even when hot, but idle is not erratic, just rough and low. Not sure if I just need to crank up the idle, or if there is some other issue. I'm hesitant to mess with the idle setting because cold start is great.

Any thoughts? I've read the brickboard FAQ's but I don't think my issue is vacuum of FPR related.

 

What year is it? A big overheat means perhaps the injectors need a good clean, try some in tank cleaner first off.
When an engine overheats badly, sometimes the fuel in the injectors can break down and leave deposits.

 

It's a '92 turbo. Cleaning the injectors is a good idea, but I thought that bad injectors would make the car run poorly when cold as well as hot.

The two possible problems that I've hit upon are a bad in-tank fuel pump and an improperly venting gas tank. Most likely it's #1 due to my noisy main fuel pump. I'm go to pull the in-tank fuel pump and test it since I can't hear it running.

 

A bad fuel pump won't cause an idling only issue. Even a bad pump will keep up with idle demands.
The in tank pump is very hard to hear running unless you take the cover off the fuel tank, being immersed in fuel, they are VERY quiet. The plastic tank also kills a lot of noise, they don't resonate like a steel tank.
The easiest way to check if one is working is to simply pull the fuel line off before the filter/high pressure pump and have someone turn the key to ignition. I don't think a pressurised tank is an issue either, it will vent through the cap or charcoal canister before pressure is even close to being a problem for running.
I suggest you start with the basics like plugs etc. If you are concerned about fuel, check the fuel pressure. Don't start throwing parts at it until you diagnose it.
Have you tried to get codes from the car? That would be my next step if I was you.
Don't try to "bump up the idle" the EFI system will just compensate and go back to the idle it is programmed to run the engine at, and all you do is end up putting the TPS out of it's idle position.

 

when my 745T's in-tank fuel pump was bad, my main pump was really noisy. new in-tank pump and the main pump is nearly silent.

if its a 92, you have OBD (onboard diagnostics, *NOT* OBD-II that you need a code reader for, instead, you read them with a blinky-box on the front side for the left wheel well). have you checked for codes?

OBD also has some useful diagnostic modes, like fuel system DTM#3 cycles the fan and injectors and some other systems with the engine off so you can stethoscope them to see if they are all clicking equally.

see the 7xx FAQ on the vcoa or swedish bricks site.

 

I wasn't aware of the codes, since the check engine light isn't on, I figured there would not be any to read. Are there still codes that I can read? vcoa is pretty much where I get most of my info, but this is a weird deal that isn't completely explained on their site.

Typhoon, you're right I hate to throw parts at the car, especially since I've done so much already. I opened the plate giving access to the in-tank pump and got about as close as I could with my wife turning the key w/o starting. I didn't hear anything, but if it's super quiet, it might be tough to hear anyway. The car has about 115,xxx miles and everything is in really good shape, FWIW

 

only some codes throw the CEL (check engine light). CEL only comes on for codes that effect smog. my car has a near permanent EGR code (2-4-1) without a check engine light, one of these days I'll dig in and find out why (probably need to the clean the EGR valve, as the vacuum acuator seems fine).

you should be able to hear the in tank pump go softly thwupp-thwupp via an ear to the gas cap with a low gas tank when you turn the key on. it only does it once or twice before you start the car, not continuously.

 

^ good to know, thanks! Once the temps climb down from 100 here I'll head to my fine workshop (driveway) and see if I can't pull some codes.

I've been driving the car with this issue ever since I bought it. Replacing the worn and torn vacuum lines actually made the hot idle issue worse--crazy.

 

Yeah, as I said, the in tank pumps are very quiet (if working correctly), they don't pump with much pressure, just supply good volume to the pressure pump.
Here's a good site for the codes:
Troublcodes.net Trouble Codes OBD & OBD2 Trouble Codes and Technical info & Tool Store. By BAT Auto Technical
As Pierce stated, only a very few codes will trigger the dash light, ones that are serious.
I know I recently found my coolant temp sensor had a corroded plug by using the diagnostics, dash light didn't come on. I was looking for a fuel consumption problem and the coolant temp sensor was it.

 

I prefer this source for the OBD-1 info on the b230f/ft LH2.4 motors....
Engine and OBD Diagnostic Codes

 

Ok, I've only tested mode 1 before darkness inhibited my progress. Pulled the 1-1-1 code so I'll wait to see tomorrow if I have any codes in modes 2 & 3. Also, just to double check, I pulled the fuel line from the in-tank pump to the main pump and turned to KPII and sure enough, got a stream of fuel. Not tons, but a good flow (my tank is <1/4).

I'll keep trying to figure this out and hoping the diagnostic tool can help.

 

Only Diagnostic Test Mode (DTM) #1 shows fault codes. the fuel and injection subsystems each have their own sets of DTM#1 codes, jumper 2 (fuel injection) and jumper 6 (ignition codes). Each subsystem also has a DTM#2 where it tests various sensors (fairly complex procedure, you have to manually activate the sensors), and a DTM#3 where it cycles various actuators (fan, injectors, EGR, etc... very handy for testing the electronics and actuators). Some cars have more jumper positions, like 3 is ABS. The DTM3 test on jumper 2 also cycles the idle control valve, so you can disconnect the hoses and blow through it to verify its working. on California cars, DTM3 on jumper 6 is an EGR test, it pulses the EGR once a second, so you can test the vacuum controller, and the EGR valve while the engine is cold.

I'm pretty sure my idle stumble is egr related, but it might be a gummed up idle control valve.

 

try changing the intake casket... My car would rough idle and loose power and cut off till I had this fixed. Not once a problem since.

 

Well, haven't figured it out yet. Went through modes 1 & 2 on the diagnostic test and all the codes came back with normal. This problem seems to have gotten steadily worse since I've started driving it (after doing the HG). When I first got everything back together, the car ran great, then over the next 100 miles or so this issue started appearing. Now it's pretty bad, car died in my driveway after I did a hot restart.

I'm not sure it's electrical. Is it possible that my FPR is stuck in the position that is pushing gas back through the return line. I don't have a way to pressure check the fuel system, but when I turn the key to II, the main pump will cycle for a second or two then shut off. To me that indicates ok fuel system pressure. I'm also not totally sure about whether it is something in the electrical system being heat affected, since I haven't had this issue occur after driving the car for a bit after a hot restart.

I'll keep looking at stuff, but at this point I'm pretty much out of ideas and just chasing stuff down. Going through the elimination process.

Thanks again!

 

afaik, the fuel pump is timed at key on, there's no electronic pressure demand. key on, pump runs 1 second or so to prime things. then when the engine turns over and the hall sensor picks that up, the pump comes on and stays on.

the pressure regulator always returns fuel to the tank, it just 'pinches' the flow to regulate the pressure. the vacuum line to the FPR is so that the fuel rail pressure is 43 PSI above manifold pressure, so at idle with the gas completely off when the manifold is at about -8PSI, the fuel pressure drops to like 35 psi. at wide open throttle (assuming no turbo), the manifold vacuum is 0 so the fuel pressure is 43psi. if you have a turbo, the pressure goes up with boost.

 

ok gotcha, so the fuel pump shoots for a few seconds automatically at key on, not dependant on pressure. Since my car is a '92, I don't think it has the hall sensor, but it still could be FPR related. I just don't understand how the heat would affect that part...

I'll keep staring, but thanks for your insight.

 

k, its probably not a hall effect sensor, I was mixing things up (my motorcycle has one of those). what I meant was the crank position sensor, which is, I believe, on the back of the engine above the flywheel, and tells the EFI and ignition when its at TDC.

 

On our 88 volvo 740 GLE we had a few strange running issues after a water pump went and it overheated. It had an aftermarket O2 sensor and they get killed fast with overheating. When I pulled the sensor out, the metal shield around the sensor was completely gone. New sensor, no more problem. One O2 sensor we had was reading fine on the meter (88's do not have diagnostics) but when we took it out, the vent openings to the sensor were completely blocked with deposits.

Russ

 

pierce...spent an hour or so trying to get the car to idle poorly in my driveway to I could go through things methodically. Tested the diagnostics in mode 3 so the car cycled through all of the various components, everything came on. Once I got the car nice and hot, the problem appeared. I may not have noticed it before (due my driving it around) but when the hot start idle problem happens, the tach drops to zero, even with the car running.

What that means is that I think you are right about the RPM or crank sensor--the one behind the engine. I'm going to check that tomorrow when its light.

wrascal--I'm also going to pull the O2 sensor as well. Not solved, but I think I'm making headway.

 

if the crank sensor was failing, the car would not run at all.

I'm thinking maybe the temperature sensor on the block. thats a primary input to the EFI to tell it how warm the engine is, so it knows what sort of mixture it needs. its on the intake side of the engine, on the block just under the head gasket, just about in the middle (so between cyls 2 and 3).