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Matt, Engineer
Category: Volvo
Satisfied Customers: 20361
Experience:  Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
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My 740 Turbo runs fine when cold but when it gets hot, runs

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My 740 Turbo runs fine when cold but when it gets hot, runs very rough, horrible gas mileage even before this new issue. Started after I washed under the hood (car was not hot when I washed it, do not think anything broke, cracked, etc.) Any ideas?

This could be a wiring issue as things tend to expand when hot and its possible a poor or partial electrical connection is being pushed apart on one of the major sensors so check the connection plug and wiring to the following;
Airflow meter
Crank position sensor
O2 sensor
Throttle position sensor

Also worth checking the connections to the fuel pump for the same reason - unplug each in turn and give them a good squirt of switch / contact cleaner - Not WD40

and also unplug and inspect the main power relays and fuses for signs of overheating or corroded terminals - so the ones for the ECU and the fuel pump

And also unplug the wiring loom connections in the engine bay one at time and inspect for corroded or recessed pins.

Check the resistance of the spark plug leads ( 13-15Kohm / metre) and replace if any doubt or high resistance -
Customer: replied 3 years ago.

I know that Volvos are famous for electrical issues and your suggestions are all good ones. However since this running rough issue started after I had washed under the hood, even tho I was careful not to try to hit the spark plugs, etc., only the engine, water from a car wash does splash. So I'm wondering if doing that would have hit something under the hood and, if so, what? Did change out the spark plugs anyway, was needed, and I am going to get some thermal grease/heat sink compound today to put between the 2 parts of the ignition control module. Any other ideas given the washing under the hood action that seemed to be the beginning of all these problems?


I'd check the state of the crank speed sensor thats on the flywheel end of the engine also the MAP ( manifold absolute pressure) sensor thats connected to the inlet manifold with a length of rubber hose

and the mass airflow sneer thats mounted in the intake pipe to the engine just after the air filter box
Matt and other Volvo Specialists are ready to help you
Customer: replied 3 years ago.

I assume when you said to check the mass airflow sneer you really meant to say sensor? Thanks for your tips. I'm thinking I really need to take it to a real volvo mechanic (hard to find one, let alone a good one in my area) to help diagnose. Today, it was doing the limp home thing after about 5 miles. Thanks for your help.

Hello Judith

yes spell check isn't all its cracked up to be

it should have read 'sensor' not sneer....