P0172 Intermittant Volvo 850T5

Having small issue — CEL comes on after 25-35 miles of normal driving. One code P0172, no codes pending. Reset, and it takes another 35mils of driving to come back on. New plugs, cap, rotor, air filter… Suggestions? Seems very intermittent so something must be on the hairy edge…

• This topic was modified 2 years, 8 months ago by  Don Bishop.
• This topic was modified 2 years, 8 months ago by  Don Bishop. Reason: improve clarity

Topic

[Richard KirshyParticipant]

A Volvo PO172 code may mean that one or more of the following has happened:

The MAF (Mass Air Flow) Sensor is dirty or faulty.
Note: The use of “oiled” air filters can cause the MAF to become dirty if the filter is over-oiled.
There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
There could be a vacuum leak.
There could be a fuel pressure or delivery problem.

[Don BishopParticipant]

Hi Richard, thanks for the reply. First, I don;’t use an oiled air filter, so we can rule that out. I did change the air filter, even though the previous one wasn’t particularly dirty or old. I use OEM Mann filters. I have had this car since it was new. Speaking of “new”, the MAF sensor is reasonably new too, and it’s a Volvo OEM part.

Of the other common causes, the code is extremely intermittent. Most P0172 code problems are persistent, not once every two or three hours of stop-and-go driving.

I am thinking this is more on the vacuum leak side, rather than the fuel delivery side. I have a fuel pressure tester, but the 850 fuel rail required a 90deg Shrader valve adapter I don’t have, the throttle cable plate prevents using a straight connector. I should have the fitting tomorrow.

Strangely, I was getting a P0456 Small Evap leak detected code. I seemed to have fixed that by replacing the original gas cap, after nearly 25 years of faithful service (-: As soon as that went away, I started getting the P0172 code. Literally, right away, after driving for 30 miles or so around town.

I am too old to believe in coincidences. Do you see any connections that might point to a common cause? I ordered a smoke tester…

• This reply was modified 2 years, 8 months ago by  Don Bishop. Reason: added detail about MAF

[Richard KirshyParticipant]

Hmmmmm interesting.
You show a small EVAP leak, which would tend to allow the engine to run richer than normal due to some amount of extra air so the ECU compensates by adding fuel.

Repair the EVAP leak then get a code for excessive fuel (or too little air)
I would be curious to see if you disconnect the battery over night and let the ECU reset.
Then run it through a drive cycle and see if the ECU relearns its fuel mapping.

Just thinking out loud.

[Don BishopParticipant]

Thanks Richard. I will give that a shot. Heading out to disco the battery. I hope I still remember the radio code….

The fuel pressure tester elbow should be here tomorrow, though I suspect that’s not really the issue — perhaps a stuck injector, I’ll probably clean them regardless.

Then the smoke-tester… Lets hope the reset works (-:

Thanks again

[Richard KirshyParticipant]

Please keep us posted.

[Don BishopParticipant]

Well, rats, that didn’t work. Right at like 20 miles the CEL came back on. No idea what to try to fix, I have to get in the car and drive 20 miles to see if whatever I do worked …)-:

[Richard KirshyParticipant]

Well it was worth a try …
Only thing I can suggest is testing fuel flow from each injector and see if one is pumping excessive fuel compared to the others.

[Don BishopParticipant]

Hey Richard, Yes, was easily worth the try, good idea. Maybe I’ll put the old gas cap on to see if the code changes back to evap.. Still waiting on the Amazon to show up. And my sense is it’s vacuum related, probably because those problems suck a lot and so does my life ..

[Don BishopParticipant]

Fuel pressure is 40pis key on, not running, about 37psi running. Revving ran between 37 and 43psi. No pulsing on idle, steady.

After 30min engine and key off, fuel pressure was still at 28psi in the rail. So I guess I can rule out leaky injectors, although I probably will take them out and clean them anyway while I’m at it.

• This reply was modified 2 years, 8 months ago by  Don Bishop.

[Richard KirshyParticipant]

That still doesn’t disprove a faulty injector spraying excessive fuel.
That test is a bit different:
https://www.youtube.com/results?search_query=fuel+injector+flow+testing

[Don BishopParticipant]

I changed the purge valve and the dried-out vacuum hoses connecting it to the intake manifold tree. Problem remains. Next up: Clean fuel injectors and clean throttle body plate. I had a problem with my 2001 V70XC caused by a sticky throttle body plate.

All suggestions welcomed (-:

[Don BishopParticipant]

I found a leak on the vacuum tree on the intake manifold by the throttle cable assembly. I fixed that and after 35 miles of driving it’s not throwing any codes. I will drive it a bit more, and if it stays off for another 50-100 miles, I will comment that this is closed. So far it seems to have been related to a leaky fuel cap which was throwing a P0432 low leak EVAP and once that was fixed, it changed to being a P0172 too rich code. After the purge valve and sealing the vacuum tree, that seems to be cured too.

I have not touched the fuel injectors at all.

I will wrap up this thread for posterity in the next day or two if the codes stay clear.

• This reply was modified 2 years, 8 months ago by  Don Bishop. Reason: clarity regarding the fuel injectors

[Richard KirshyParticipant]

Glad to hear you may be on the right track.

[Author]

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