My 940 Turbo starts great when cold. Runs strong for about 5 minutes and then suddenly seems to lose gas while applying throttle.

If I back off the throttle or put in neutral, the engine stays alive. However, if I keep trying to drive, the engine eventually dies altogether.

When in this state, the engine will not start at all (acts like no gas) until it has a chance to cool off. After maybe 30 minutes, it will start - but just barely. I found that i can pump the pedal rapidly to keep it going and even put it in drive to limp it along.

My trusted mechanic has tried to zero in on the AMM so far struck out after trying the following:
1) Replaced the intake hose from the AMM to the Turbo (old one was really bad)
2) Replace oxygen sensor
3) Swapped the AMM with a known good one from another car

I am getting a 3 - 1 - 2 code "Signal missing for knock-related fuel enrichment" from the on-board diagnostic. Not sure if that is helpful or not.

Any clues from seasoned RWD mechanics?

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Thanks for all the posts and suggestions from spook and mapleleafer. Much appreciated. Here is an update ...

New behavior:

After the last time it died and i was barely able to limp it back to the mechanic, I generally have not been able to get it started normally. It coughs and sputters..

I found a "traveling" certified volvo mechanic who agreed to come by and take a look. The car magically started right up initially. I told him that I'd probably have to drive it around to get it to act up. Well, I didn't even have to back it out of the parking lot before it coughed, sputtered and died (like it's losing gas).

His initial suspicion was a faulty fuel relay - negative. Replaced the knock sensor - negative. Jiggled wires and modules in a few different places testing for shorts - negative. He seemed to feel the pumps were working (from listening for them). Didn't bring a fuel pressure gauge, so wasn't able to verify anything there. Checked the codes and discovered 2 - 2 - 4 "Engine coolant temp (ECT) sensor signal absent or faulty". He now suspects something there but is researching it.

Meanwhile, I happen to go to church with ANOTHER volvo mechanic and I've gotten him involved. He's convinced that it is a faulty ICM - Ignition Control Module. Opinions?

Tomorrow I will attempt to swap modules with my other 940 (same part#). I'll post results.

The knock sensor input may cause late ignition timing but it should NOT cause the symptom you described. You absolutely need to monitor fuel pressure to find your answer. I'd put money down that it drops at least 2-3 PSI simultaneous to your power loss. Given the repeatability, I'd say the pump itself is not the most likely failure. Write back when fuel pressure is confirmed (no guessing allowed).

Ok no guessing! Looks like I can get a pressure gauge relatively cheaply. I'll pick one up and let you know.

Thanks!

I would think that your mechanic should have one. It'll have to be adapted to read regulated pressure and placed where you can red it while driving. If you disconnect and plug the vacuum hose at the pressure regulator (front end of the fuel rail), you'll have a stable reading regardless of boost. Either 36.75 PSI (2.5 bar) or 44 PSI (3.0 bar) depending on FPR installed. If you see it drop even one PSI from normal - you've zeroed in on the basic cause.

If it passes this test, reconnect the vacuum hose and be sure the pressure rises during turbo boost (refer to dash gauge).


If it passes both of these tests... then it is NOT a fuel problem.

Question; does the turbo boost gauge go well into the boost zone under hard load, or more like "barely"? Do you see any black smoke out the back when you stomp it?

ok. I'll give my mechanic a call and see if he can hook it up as you suggested.

Answer to turbo gauge / black smoke:
1) Turbo gauge needle goes into the black "boost zone" but I wouldn't say very far into it
2) No, there is no black smoke

This MAY be your answer. See if you can duplicate the problem at zero MPH by putting car in D, holding the brake down firmly and flooring the accelerator - does the RPM pickup normally for a second or two and the the engine "chokes"? Have an helper watch the tailpipe at the same time. Do this in a parking lot or open area.

If it revs strongly at zero MPH, have your helper follow in another car and watch the tailpipe for black smoke under accel.

A small pressure leak in the hot side of the turbo piping will cause all your symptoms, but the loss of power is caused by the opposite of what you posted in your original description.

It duplicates precisely that way. In neutral, it revs perfect. Put in drive with the brake and the engine chokes/dies. I'll have someone check for the black smoke.

Assuming this is the problem can the turbo just be re-seated with new gasket?

By "opposite" did you mean the knock sensor code?

DONT jump to conclusions, it is not the turbo itself. It is one of the intake hoses blowing open under pressure. The most common one is at the throttle body. Tears usually happen on the bottom side (where oil eats the hose away from the inside) and at the edge of the clamp. All four hoses on the pressure side are suspect.

By opposite, I mean that the engine is flooding with fuel - not starving for it. The reason is that fuel is metered based on the air passing through the AMM. A pressure leak means some of that air did not enter the engine for combustion. Short version = less oxygen than calculated therefore excess fuel.

At this point, it seems that your sensor code is 100% unrelated. After fixing the intake leak, clear the codes and recheck. There may be something else to fix but it's not the cause of this problem.

I finally was able to observe the behavior with a fuel pressure tester.

The car had been sitting a while, so of course it started right up. Pressure @ idle around 35 psi. Fluctuates slightly reving in neutral.

Put it in drive with the brake. Pressure hits up to 45. Everything seems fine.

I began to drive it around the parking lot (while observing tester) and it did not take long before it started to lose power / gas. Pressure down to 12psi in neutral.

At this point reving in neutral or in drive causes the engine to start to fail and pressure consistently stays @ 12.

Pressure regulator?
Main/secondary fuel pump?

Now you know why I said this was important information. If your mechanic had done this early on (and based on the symptom, he should have), you would have saved a LOT of checking, guessing, testing, asking, replacing, getting stuck, walking home.

Pressure regulator is out of the picture, this is a delivery fault. My instinct has me at the same point as I started - main fuel pump related. Being so consistent puts a few other items a lottle lower on the list. However, if I was working on this car, I'd monitor the voltage at the pump terminals to be sure that the voltage isn't dropping due to heat and load showing a wiring weakness. You can do this by taking two wires, stick a bared end into the push-on connector and replace onto each electrical post on the pump. Put a voltmeter on the other ends to monitor pump voltage. If the pump voltage remains over 11V and pressure drops, pump itself is bad. Because of the consistency, I'd say that one winding of the armature is grounding when heated and causing a significant drop in motor efficiency. It's one of the few explanations that logically explains your symptom without ghouls, goblins, ghosts or maybes.

Relay, athough possible, usually fails because of bad solder connections inside and cause unpredictable LOSS of fuel pump, not low pressure.

Intank pump can be checked separately but high fuel tank level almost always covers its failure. You can disconnect the big hose to the main pump, put a bucket under the hose, crank the starter for 15 seconds. If you have a cup or more of gas in the bucket - it's good. Faster test; if the car runs the same with a full tank, this isnt the cause - even if the pump is bad.

Try one other thing before going further - on the itsy-bitsy chance that the tank vent is blocked, loosen the fuel cap right after the pressure drops and recheck it.

So you don't have to ask what some of this means; if voltage and fuel cap checks don't reveal anything, then the main pump is your answer.

Not an expert on the flow of fuel in this car, but could it be a dirty blocked fuel filter? Seems to be related to heat, so the pump is suspect, and Volvos rarely stop from fuel filter blockage, but if it's over 50K miles and the pump seems OK, I'd change it.
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That's more guessing, and not even a good one. If the filter were partly blocked, the symptom would be load related and always present. The car could not run OK cold (in fact it would run worse during cold enrichment) and the pressure would recover at idle.

CLOGGED FUEL FILTER!!!

At least so far so good. I'm taking short trips with this baby till I'm sure.

I've owned the car for 30K miles. The filter is rated for only 50K/60K I believe. Not clear if the prior owner ever changed. A lot of black stuff does come out when shaken.

Theory on behavior? Well, my guess is there is a lot of sediment in that filter. When it sits a while, the sediment settles. This explains why the car starts right up and drives fine initially after waiting long periods of time. Secondly, the sediment gets stirred up and clogs the filter - but to different degrees each time. This explains why sometimes it won't start at all and other times it would barely chug along.

Kudos (vindication) to RUNWLD2 and another friend was urging me to try this before replacing the fuel pump. Kudos to MAPLELEAFER for urging me to do the fuel pressure test (had to buy a tester).

Lessons learned?

1) Start with the cheap silly stuff: fuses, filters and make sure you have gas.

2) Don't guess! Use the right equipment / diagnostic to properly test and narrow down the problem.

Dear aconn7,

May this find you well. OBD Code 3-1-2 means, "No knock enrichment signal from ignition control module (ICM)".

The wiring diagram shows two wires - one green (signal) and black (ground) connect the knock sensor (mounted on the engine, ahead of and below the intake manifold) to the ignition module (under the dashboard, above the driver's footwell). These wires pass through Connector C132, which is under the brace for the driver's side strut tower. The black wire goes to Connector 12 and the green wire goes to connector 13, on the Ignition Control Module.

The knock sensors rarely fail. I'd inspect the wires. Corrosion or damage to a wire could disrupt the signal.

I do not have the trouble-shooting manual for the fuel injection system. One of Board's bonafide experts will surely comment.

Hope this helps.

Yours faithfully,

Spook

I'll second Spook's suggestions:

Start by cleaning and inspecting the electrical connection at the knock sensor, then cleaning the big multiplugs hiding under the drivers side strut brace. There's a grey one and a black one--open up and clean the electrical contacts on whichever one includes the fat green wire from the knock sensor.

I would not start playing with expensive things before eliminating these two points as possible problem areas.

John

Those are great suggestions. Is it clear that the knock sensor issue could actually be causing the primary problem? I'm just leery of heading down the wrong path on this.

Dear aconn7,

May this find You well. I'm not sure that the knock sensor is the culprit. However, before you replace any more parts - randomly - inspecting and cleaning the knock sensor circuit is prudent and costs almost nothing.

You could get a good used knock sensor at a salvage yard for small money: a brand new one is about $40. The same knock sensor was used on post-89 240s & 740s and 940s with B230Fx engines.

Hope this helps.

Yours faithfully,

Spook

Ok, you make a great case. I'm going to be getting the wagon back from the mechanic tonight (if I can limp it home that is) and check this out tomorrow. I'll post my findings.

BTW, kudos to my mechanic. He knows I don't want to spend a lot of money on this. So even though he hasn't found the problem himself he's not charging me an labor for his trouble so far.