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Stumble/Hesitation/Misfire Help

M

MSGGrunt

New member
Joined
Feb 13, 2018
Location
Western Massachusetts
When on first start and a cold motor my 240 has a hesitation/stumble/misfire. This is at idle and when driving. This does seem to go away once the motor is fully up to temperature, say after 15 or 20 minutes of driving. Just wondering where I should look.

- 1991
- 125,750 miles
- New Volvo plugs, cap rotor and plug wires (hesitation was present before and after these)
- New air filter
- New crank position sensor
- The hot air tube from the exhaust manifold to the air box was removed by the PO. The airbox has been capped

My thoughts are:

- MAF
- Dirty or faulty injector
- Bad sensor of some sort
- Dirty fuel filter. (but I would think this wouldn't be temp sensitive)
 
I would check for some sneaky vacuum leaks. Stuff like the hoses from the throttle body down to the charcoal canister. The corrugated air hose between the maf and the throttle body. It likes to crack on the bottom where you can see it. Intake gasket and injector seals.

Also on the air box. If it is capped off from the hot air. You must make sure that the flap isn't covering the cold air input. That will restrict airflow to the engine and cause this kind of issue. I remove the whole bracket with the air thermostat and flap. Then I direct the hot air to the front so you have two cold air intakes to the air filter box. Or just cap off the hot air with the bracket for the flapper removed. It's just snapped into the edges of the air filter box on four places.

Another check would be the coolant temp sensor but that usually throws a code. I would still check for codes though. Some of the codes don't turn on the check engine light.
 
A cold motor needs a slightly richer mixture to operate satisfactorily. Lean operation can result in misfires until such time as the engine warms up. Once warm, your engine may still be running on the lean side; but, its not as sensitive.

Check your 02 senor voltage. The OEM narrow band won't tell you the AFR; but, it will tell you whether you are running consistently above or below 14.7 AFR.

If the O2 sensor voltage indicates that you are running consistently lean during start up, all of the things you mention could be possible causes including the aforementioned manifold leaks anywhere downstream of the MAF. A dirty MAF will also mess up fuel mixture.
 
ZVOLV said:
The oxygen sensor does not operate until it is heated up.
Click to expand...

Correct. However, electrically heated O2 sensors will come up to temperature fairly quickly, typically long before the engine block is up to temperature following a cold start on a cold day. The voltage can give you a gross read as to whether fuel mixture is wanky during that period when the engine is still cold.
 
I would smoke test the intake.

A torn accordion hose can cause lots of issues. Make sure it hasn't rubbed thru against the strut tower.
 
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