SOLVED 96 Volvo 850 rpm's "governed" at 2.5k Topic is solved

[JimBee]

Gentle acceleration moves easily through the gears and can cruise at highway speeds with a light foot. Hits rpm limit at ~2.2k.

Attempt to downshift or press beyond 2.2k engine stumbles, light backfire, seems like it would stall if accelerator held down. Exhaust is clean. Starts normally every time. Idles smoothly, although has always been a little high ~900 to 1k.

Initially, I thought this was a failing fuel pump. New fuel pump hasn't fixed it. Also has new Bosch TPS.

EDIT: thanks to the MVS wizards who pointed out where I should be looking. Neil, abscate and Erik filed some clues; Matt hit the bullseye recounting his own 850 turbo experience. On mine, the lower hose on the intercooler had come loose and was about to fall off the nipple. A nice 5 minute fix and now back to full strength

[scot850]

What engine have you got in your car Jim? I would suggest MAF, then intake or vacuum leaks. With a turbo, look for split intercooler pipes and intercooler.

Neil.

[erikv11]

Yeah, that doesn't sound much like a fuel pump problem. There are exceptions but they are uncommon - for the most part fuel pumps either work or they don't. This one would be simple to test, the usual check-pressure-at-the-rail.

I too favor the the intercooler hoses if its a turbo, or the vacuum leak idea in general, but it would help to know what you're driving. And good news is the MAF test is free, no throwing parts at it!

Is the exhaust flow unobstructed?

[JimBee]

This one is the 2.3 turbo 855. I forgot to mention I've p0172 and 0440 codes for a while but I hadn't noticed that they affected anything. I know the 1/2" ID hose that connects from a steel line (from the filler pipe?) passes in front of the fuel pump then under the floor pan is broken. Might that be throwing the evap code? Could that affect the engine speed?

[abscate]

Any unmetered air into induction will upset the balance of what the ECU thinks it is getting and what the engine actually gets. This means it trys to add fuel to the mix until it runs out of trim room - and then the code comes P0171/2

[scot850]

If the pipe is split, it might be fairly easy to sleeve it to at least see if it makes a difference.

MAF test mentioned I am assuming to disconnect it and see if the car runs better. I'm not convinced the MAF is the issue, but a simple test like this at least should eliminate it as a possible cause.

Neil.

[JimBee]

I'll try disconnecting the MAF.

Which pipe might be split?

[matthew1]

Jim

Almost certainly unmetered air getting in between the air box and the intake manifold*. This sounds just like my 850 when I blew the intercooler. It cracked and allowed air to get in.

*EDIT: changed "intake" to "intake manifold" for accuracy

[JimBee]

Matt & others. So the excess unmetered air overwhelms the amount of fuel the ECU thinks should be allowed into the injectors (based on MAF input) exceeding the upper explosive threshold of the air/fuel mix so combustion is sort of suffocated?

Will disconnecting the MAF test for excess air?

[erikv11]

The unmetered air causes the engine to "run lean" i.e. based on the MAF readings the ECU is supplying less fuel than should be given the amount of air in there. Really it is the opposite of suffocation (not enough air), it's more like the engine is hyperventilating (too much air, not enough fuel).

The MAF test only checks for a bad MAF, it is not a test for unmetered air. When you unplug the MAF, the ECU realizes it isn't there so it provides a default amount of fuel per throttle position. This amount will make the car to run a little rich, to protect the engine, which if you think about it could possibly also counteract the unmetered air. But from the sounds of the (potential) leak here, unplugging the MAF would not likely provide enough fuel to make up for the extreme lean conditions that are causing the car to stumble so badly.