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1988 740 turbo - no power - backfires
06-18-2010, 12:29 PM (This post was last modified: 06-18-2010 12:30 PM by ksmith.)
Post: #1
1988 740 turbo - no power - backfires
I have an 1988 740 turbo wagon, I replaced the head gasket but now it has no power, backfires and hesitates. Before I replaced the head gasket, the car ran fine except during the summer when it was hot out., it would lose power, hesitate, backfire ect after you drove about 30 miles or so. But in the winter it did fine even after a long drive (ie 80 miles +) When I stated to lose water and fond it in the oil I replaced the head gasket. I also replace all vacuum line and the fuel filter. Any ideas? And I can hear both fuel pumps running though the tank pump is very quite.[/size]
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06-18-2010, 09:54 PM
Post: #2
RE: 1988 740 turbo - no power - backfires
I think we need to start with basics and ask the obvious..... are you sure the cam and ignition timing are correct after the head gasket job? Are you sure the turbo is able to spin free and is not binding? Coolant in the oil can play strange tricks on moving parts.

Couple more thoughts...... if this was a long term project are you still using winter grade fuel?

If this has the old style Bosch air flow meter (mechanical) with the flapper, is there an air leak between there and the manifold? The air flow meter can also lose it's calibration with age, which may be something I can walk you through. (cheater's fix)

Part of me wants to say it sound s a bit like a vapor lock problem but if the fuel pressure is right it may be a lean mixture for another reason, like running hot or partially clogged injectors.
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06-23-2010, 08:04 PM
Post: #3
RE: 1988 740 turbo - no power - backfires
That is a good place to start, yes I triple-checked the cam and ignion tion both correct. Turbo spins freely, no leakage in the intake air system. no idea on how to calibrate the mass air flow meter. In Florida we don't use a winter grade fuel, never gets cols enough. Vaper lock is something I thought of, but now the problem is at all times even when the temp is cool outside. Was thinking maybe the Chargh air temp sensor, but have no idea where it is.
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06-23-2010, 09:36 PM (This post was last modified: 06-23-2010 09:37 PM by Rupe.)
Post: #4
RE: 1988 740 turbo - no power - backfires
If this is a fully electronic air flow meter (mass air flow) you should see no moving parts inside, in which case the air temp sensor is usually part of the air flow meter. There's nothing you can do but replace it if it's bad. OTOH, some have the temp sensor mounted in the intake boot so you will need to look around.

BTW, without knowing the exact fuel pressure you are just guessing. Low pressure can cause both vapor lock and lean backfire through the intake. Recalling back to that vintage the typical fuel pressure should be somewhere around 45 - 55 psi. A bad throttle switch can also cause a lean mix at "tip-in" which is when you first hit the gas. That is something you can check with an ohm meter and watch the switch make / break just off idle. (around 1000 rpm) there may be a 3rd contact on that switch for "wide open" which goes extra rich. Checks the same way but at full throttle. (engine off is easier for this) This will usually not set a trouble code.

Sorry I can't be more specific as it's been 20 years since I worked on a Volvo.
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06-25-2010, 09:58 AM
Post: #5
RE: 1988 740 turbo - no power - backfires
I need to check the fuel pressure see what it is. What if the charge air pressure sensor is bad? it makes no differance on how the car runs if it is pluged in or not.
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06-25-2010, 10:09 AM
Post: #6
RE: 1988 740 turbo - no power - backfires
That could cause problems but again it's been 20 years since I worked on one of these and I don't have a specific test for it.

Recalling that's got a rather large vac line (for fast response) and disconnecting it should change the way the engine runs. OTOH, if there's not enough fuel pressure you could be barking up the wrong tree because it's covering up another issue.

My best guess tells me if there's no trouble codes you need to concentrate on the basics first and fuel pressure generally doesn't set a code.
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