Diagnosing PO171 and PO174 for 1998 GMC Jimmy

ROYCAR71

MEMBER
1998 GMC JIMMY
125,708 MILES

How do I diagnose po171 and po174 after mass air flow replacement. I had mass air flow sensor dianosed by mechanic and I replaced to save money. Car drives normal, but has rough idle when fuel tank is at 3/4 to 1/2.

P.S. I have included pictures of freeze frame data.

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5 Replies

WRENCHTECH

MECHANIC
20,758 POSTS

All that data shows you is what you already know. The system is very lean for some reason. You need to start out monitoring the fuel pressure with an accurate gauge and check it very thoroughly for any vacuum leaks or leaks in the intake snorkel. The fuel pressure can never go below 55PSI and should max out near 66PSI under acceleration.

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Thursday, July 4th, 2013 AT 5:05 AM

ROYCAR71

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354 POSTS

I blew cigar smoke into booster hose to see where smoke might come from. I see smoke come from air intake tube/resonator. Is this normal?

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Friday, August 9th, 2013 AT 9:38 PM

WRENCHTECH

MECHANIC
20,758 POSTS

You're been watching too many Scotty Kilmer videos. You'll get sick attempting to blow enough cigar smoke to find a vacuum leak. Of course air will leak out the throttle body. That is the air intake point and would have to be sealed off in a real smoke test, although any real air leak in the snorkel between the throttle body and the MAF will cause lean codes.

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Saturday, August 10th, 2013 AT 2:45 AM

ROYCAR71

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354 POSTS

Thanks, I will try with one of those $29 smoke machines off ebay.

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Saturday, August 10th, 2013 AT 3:13 PM

WRENCHTECH

MECHANIC
20,758 POSTS

$29?
You want to be very careful. The EVAP system can only deal very small amounts of pressure. I paid $1300.00 for my smoke machine because it has very precise pressure measurements.