760 Cold Idle/Running Issues

I have a 1985 760 Turbo I recently purchased. When I test drove the car, it started eagerly and ran really nicely right up until I gave it a boot full of throttle to see how she boosted. Boost was great, but as soon as I let my foot off the throttle (from 100% open to snapped shut), the car died with all warning lights on. I pulled over and it immediately started again as if nothing had happened. I knew it was going to need diag, so I talked the guy down and drove it home- it didn't complain for the rest of the night.

The next day I go out to start it (outside temp about 40F), and it fires up right away and idles at ~1000RPM with no hunting. Several seconds of idling later, the RPMs slowly drop and the engine dies. You can restart the car a dozen times and it will be the same- fires up strong then slowly kills itself. If I hold my foot on the throttle and let the car warm up even a little bit (5-10 minutes), it's as reliable as the day is long and will start/restart with no problem.

I've cleaned out the IAC and repaired the split outlet hose I found, but the problem remains. So TB hive mind- what next steps should I add to this list?

1. Remove intake manifold and throttle body, clean and adjust to factory specs
2. Adjust or replace throttle position switch
3. Fully dismantle IAC and clean brushes etc. per 700/900 FAQ

Any ideas are much appreciated- I haven't had an LH2.2 car in a while, and I'm not sure what else would affect cold idle.

BrickBorg612 said:

...let my foot off the throttle (from 100% open to snapped shut), the car died...next day I go out to start it (outside temp about 40F), and it fires up right away and idles at ~1000RPM with no hunting...Several seconds of idling later, the RPMs slowly drop and the engine dies...

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Cold Start - LH 2.4 (I know you got LH 2.2) for several seconds or more will enrich mixture during a cold start.

Higher RPMs - When throttle plate is closed, if intake has air to suck on, it will suck air, and your fuel/air mixture is not balanced.

Check brake booster for a vacuum leak...intake manifold may be sucking air...etc

Thanks for the reply- I ordered a new intake manifold gasket, vacuum lines, and TB gasket from IPD since there seem to be a number of leaks in the area. Appreciate the heads up.

Check the condition of the underhood wiring for breaks and being very brittle. I've replaced the whole enchilada with a new harness on the current '87 765T and on a prior 740 non-T.

whalepirot said:

Check the condition of the underhood wiring for breaks and being very brittle. I've replaced the whole enchilada with a new harness on the current '87 765T and on a prior 740 non-T.

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I was planning on doing so in the near future just as preventative maintenance- where did you get your harness and were you pleased with the quality? I was going to order one from Dave Barton's site.

Your model year may precede Volvo's production years with the crappy, eco-friendly wiring.

BrickBorg612 said:

pleased with the quality? I was going to order one from Dave Barton's site.

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Yup, him. He's awesome; knowledgeable and helpful even with some issues unrelated to the product I bought.

The turbo is more work than the non, but following the instructions, it went well, IF taking longer than I'd anticipated. Suggest taking pleanty of pics for later reference, especially if your car is non-stock. There's variance in the model years, too, and those pics will ease the install, and yes, it IS easier to pull the intake off.

Here is an update for posterity in case folks are chasing similar issues. It seems to have been a combination of the vacuum leaks and IAC being toast. The car now idles rock solid on its own when cold (no throttle pedal artificial human enrichment necessary at idle). To get to this point, the following was done:

1. Replaced IAC after dismantling the old one and finding it completely hosed (this fixed 95% of the problem).
2. Replaced failed throttle body gasket and idle air screw gasket.
3. Replaced split vacuum hose from intake to fuel pressure regulator.

The car now idles great but runs poorly until it's up to temp (lack of power and bad gas mileage), but at least its a start. When it's at temp, there is a notable lack of power until the turbo kicks in, but it boosts and moves fine after that. Next steps are a smoke test to find the rest of the vacuum leaks, fixing the intake manifold gasket (you can see where it's been blowing for a long time under boost), and checking engine timing. The timing belt was done by a shop that I don't really trust after seeing some of the bodges they did on this car trying to get it to idle for the previous owner, so I feel it prudent to check.

BrickBorg612 said:

...runs poorly until it's up to temp....

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Throttle plate must be set to OEM specs, and idle switch must be set correctly, and plugged in.

Circling back with this, I've been chipping away at what is clearly a compound problem. Any ideas to add to my plan below are welcome, I'm running out of feathers in my diagnostic cap.

Initial Symptoms:
Car would refuse to idle on its own until it was up to temperature. Would immediately restart and die without throttle application. When car was warm and driveable, noted lack of power and poor gas mileage. Periodic stalling when driving, immediate restart OK.

Diagnosis:
Noted IAC was bad. Replaced IAC and hoses, adjusted throttle switch properly, replaced idle air screw gasket and throttle body gasket- car now idles on its own. Car now drives very poorly cold- no power, often misfiring. Car drives ok when warm, but still lacks power. Car will still stall periodically, even when warmed up. Temp gauge stays in the bottom 1/4th while driving. Car pulls almost no vacuum when cold- needle almost touching the yellow section of boost gauge at cold idle.

When driving car warmed up, started to stumble, then died. Would not restart, even with throttle application.

Replaced crusty immobile thermostat. Car now gets to proper temperature. Still experiencing stumbling if driven cold, car still stalls periodically. Stall can occur at any temperature, any RPM, and at any speed.

Adjusted AMM to correct specs per Brickboard FAQ.

Timing belt found to be 2 teeth off. Replaced timing belt and timed correctly. Adjusted ignition timing after belt replacement- shop who did last timing belt had the distributor fully retarded. Noted that car died twice when removing the timing light clamp from plug wire #1 during timing process. Car still eager to start and impossible to drive when cold, but when warm, it has the appropriate level of power. Fun!

At this point, my wife started to use this car as an emergency daily driver. Car starts and idles on its own regardless of temp, starts easily in temps as low as -20F. Periodic stalling continues regardless of temp or speed. Happens maybe once per week. Car always restarts.

Evidence of intake manifold gasket failure is present on the head- replaced intake manifold gasket. Boost seems a bit stronger, no change in cold behavior. Noted oil leak from rear of valve cover/distributor area. Suspect rear cam seal- to be replaced. Cold vacuum issue still present, but a hair better. Car pulls normal vacuum when warm.

Today:
This morning I went out to start the car in +25f temps and it seems we are back to square one- car will not idle on its own. When throttle is applied to hold idle at 1500rpm, car seems to be misfiring badly. If you release the throttle gently after this, the car will idle at 400-500 rpm. Died again shortly after. Did no further testing, it was 5am and I had to leave for work.

The Plan:
Replace plugs and wires (the car dying when taking the timing light off makes me think this needs attention asap), clean distributor out and replace rear cam seal. Find someone with a smoke machine to test for smaller vac leaks- my homebrew cig method and the carb cleaner test showed nothing leaking but the intake mani, throttle body, and idle screw gaskets which I replaced. Re-adjust base idle when this is done. Beat my head on the workbench as needed.

Other ideas? I'm open to anything at this point.

Did you check fuel pressure regulator for fuel leaking into vacuum hose?