Trouble with my 1999 C2500 Suburban again...P0174 code

[travis]

I keep reading a lot of references about a bad MAF sensor. My voltmeter here at home doesn’t read Hz so I will bring one home from work tomorrow to check that. Then I will swap O2 sensors side to side and see what happens.

I appreciate the help. This is outside of what I normally work on but I am learning as fast as I can.

[Dirtybob]

You should be able to unplug the MAF and see if it runs better (has worked for me before on a 2000 GMC 1 ton 5.7L).
Also, do you have a way to reset fuel trims? Wouldn't hurt to do so in between tests.

[travis]

I was wondering if there was some way to reset the fuel trims, as the ST’s look better but the LT’s (especially on bank 2) seem to be locked. Maybe I haven’t driven it enough yet since fixing the vacuum leak?

I did unplug the MAF sensor yesterday...it bogged down for a few seconds then went right back to running like it did with it plugged in. What does that mean?

I really can’t see multiple problems happening at exactly the same time, unless running it lean for a few miles damaged something yesterday.

[BLstangin]

Most likely is the bank 2 pre cat oxygen sensor failed and stuck lean so the ecm is compensating trying to get the fuel trims back in order as you can see by the LT fuel trim at 25%, thats the max it will go, it kicks the lean code at approx 22.5% So whats actually happening is the engine is so rich on that bank that it starts to miss, it is probably not actually lean, but is rich as the ecm is adding fuel to try and compensate for what it thinks is a lean condition, replace the bank 2 precat oxygen sensor, clear the codes and report back. The long term fuel trim will come back fairly quickly once the readings are correct. Its probably not an induction problem because almost always if there is a leak in the induction side (vacuum hose, gasket leak, etc.) it will affect both banks since they are in a common plenum at the intake, and you will get a P0171 and a P0174 code together. The only other thing would be a horrible exhaust leak on bank 2 which should be passenger side before the sensor, but it would have to be pretty bad, or an injector not firing, this can be ruled out fairly easily though with a good scan tool and an injector balance test.

[Dirtybob]

I did unplug the MAF sensor yesterday...it bogged down for a few seconds then went right back to running like it did with it plugged in. What does that mean?

means your MAF is probably ok.
not too likely that a bad MAF would only affect one side of the motor anyways...

[learner1]

The fuel control will go rich at WOT. Just monitor the upstream 02 with your scan tool during WOT. The 02 should peg at .9 volts during WOT, if it doesn't either the sensor is bad or you have a fuel delivery issue.

[travis]

Another unproductive day

Swapped O2 sensors side to side, trouble didn’t follow. Went over everything again looking for vacuum leaks, tried capping the few vacuum lines off, no change. Then got to thinking that maybe the vacuum leak killed or caused the death of a spark plug (causing a misfire and possibly making the O2 sensors read a lean condition), so I pulled the plugs on bank 2 and 3 didn’t look bad at all, but one was black with fuel. Replaced all 4 plugs on that side...and still no change

I haven’t lost any water, but I guess the next step is to check the intake gaskets...

[Dirtybob]

I would suspect a stuck/leaky injector (only 1 fouled plug)
perhaps a fuel pressure leakdown test is in order.

[In-Tech]

Again a free swap to another cylinder to pinpoint.

[BLstangin]

Also if u unplug both upstream sensors do the readings match? Maybe you have a broken wire in the harness between the pcm and the connector

[midnightbluS10]

I would suspect a stuck/leaky injector (only 1 fouled plug)
perhaps a fuel pressure leakdown test is in order.

'99 should be CSFI, with 1 central injector and 8 poppet nozzles(pictured above) unless it's been updated to the redesigned spider with 8 mini injectors(pictured below).



It could still be a possibility that one is clogged. I believe you can even still do a cyl power balance test(or whatever it is) on those.

[travis]

I would suspect a stuck/leaky injector (only 1 fouled plug)
perhaps a fuel pressure leakdown test is in order.

99 should be CSFI, with 1 central injector and 8 poppet nozzles unless it's been updated to the redesigned spider with 8 mini injectors.

It could still be a possibility that one is clogged. I believe you can even still do a cyl power balance test(or whatever it is) on those.

I upgraded to the pod/8 injector setup a couple of years ago when I did the intake gaskets. I’ve put maybe 10k miles on it since then. I still don’t think the intake gaskets are bad but I’ve been wrong before.

After thinking on it some more, I really doubt that the rubber hose I found partially disintegrated that connects to the water valve was more than just a tiny part of the issue. That valve appears to just control water flow to the rear heater core, and I haven’t used it in a long time.

I think tomorrow I will try putting a timing light on each plug wire to see if they are all firing. I put plugs and a new distributor cap on it when I did the intake gaskets, but the plug wires on it have been there since I bought it. It’s a shot in the dark but easy and free to check.

A power balance test...not sure what that is, but I’ll look it up.

I pulled 1 of the plugs on the drivers side and it looked fine, but I’ll check the last 3 just to be sure. I did pull 1 plug wire on the drivers side to see if it would register a misfire or lean code on bank 1 but it didn’t???

I did end up replacing both O2 sensors on the passenger side because both read erratically on either bank, where the ones on the drivers side read normal “sine wave” looking curves. It made no difference. Based on an earlier post though I think the drivers side cat is bad as I get nearly as wide of a post cat O2 reading as I do on the pre-cat O2.

I think there may be multiple issues at play here which isn’t helping at all

[travis]

The fuel control will go rich at WOT. Just monitor the upstream 02 with your scan tool during WOT. The 02 should peg at .9 volts during WOT, if it doesn't either the sensor is bad or you have a fuel delivery issue.

I tried this as well. It didn’t change the range readings, just oscillated faster. And caused some light popping out the exhaust.

[travis]

I think this engine is pretty much done

Pulled plugs first thing today...4 plugs on bank 2 have maybe 15 minutes run time. All look ok except cylinder 2...the plug is wet with fuel and already turning black. Tore into the upper plenum to investigate...the front half of the intake was covered in thick soot and random spots of a thick sticky tar like substance. The back half of the intake is wet with oil and less sooty.

I fully cleaned the plenum area when I did the intake gaskets a couple years ago, and it didn’t look this bad back then.

Sigh....