Mechanics

CHEVY PO171, 174 CODES, CRACKING #2 PLUG, MI

1999 Chevrolet Silverado

Engine Performance problem
1999 Chevy Silverado V8 Four Wheel Drive Automatic 134000 miles

Well, bought this truck, running wonderfully with about 123k miles, decided to do a routine tune-up.

Installed ACDelco iridium plugs, gapped to.60 but later found out they were factory gapped to.40, so fixed that. Installed new air filter, fuel filter, autolite wires.

Roughly a week later my engine light came on, and went off, then a week more and it stays on. First it was bank 1, then both days later. Same 2 codes every time 171, 174 lean both banks.

Since the code has appeared, we have inspected vacuum lines, ran fuel pressure checks, 54-55psi key on, 55-60 under load, drops momentarily below 54 from dead throttle drop. Cleaned and inspected most sensors, installed a new intake gasket torqued per GM specs, inspected FPR, clean, Pulled EGR, cleaned surfaces, inspected operation, replaced gasket. Replaced MAF with rebuilt. Scantools show the MAF and O2's working correctly.

I can clear the codes, and the pendings pop up roughly 3 minutes into an idle, with the light comming on soon after.

Not long after the codes stuck, I started noticing a miss under load, usually after the truck was warm and driving steady around 30 minutes. Pulled plugs, found a crack in the upper porcelain of cylinder 2, replaced with old plug I removed, week later it cracked also. Crack begins about 1/16" down from the top of insulator and stops about 1/4" from the bottom. The old plug I re-installed cracked twice, same spot, about a mm away from the 1st crack.

It's no doubt that this crack is what's causing the miss, but what's causing it to crack?

Also, any ideas on where I can go with the lean codes. I want to take it to a dealer, but if I can try something myself that I haven't already, I will.

I am driving myself crazy working on this thing. It's still new to me, so I have to get it back to working correctly. Any suggestions are appreciated!
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Stumasters
January 22, 2010.




Autolite wires on these engines will not work. Only use A/C Delco wires.
The cracked insulator is probably the result of overtightening the plug and the use of autolite spark plug wires. The plugs should only be tightened to 14 ft. Lbs. And the wires MUST be A/C Delco or Accell Performance -- the spark plug wires should cost around $120 bucks or so.

Tiny
Pear69
Jan 22, 2010.
Yes, I was worried about the overtightening myself, so when I installed them this time around, I was very gentle, and just snugged them.

I will replace the autolite wires with my old stock delco wires and see if there is a change in the lean operation.

If so, I will be sure and order new OE ACDelco wires, considering these made it over 10 years without problems.

Tiny
Stumasters
Jan 22, 2010.
The wires should have about 30000 ohms of resistance (10000 ohms per foot). Test them to see if they pass spec. As you test them be sure to bend them around a little.

Tiny
Pear69
Jan 22, 2010.
Also, overtightening the plugs (other than the obvious physical over torque cracking) allows too much heat to be transferred to the porcelain of the plug.

Tiny
Pear69
Jan 23, 2010.
Well, swapped wires back to origionals today and drove about 10 miles. At first I thought the issue was fixed, simple as that, but then a P0174 pending popped up. Drove another few miles and then the P0171 pending popped up as well.

Another drive cycle will throw the CEL back on.

I will say that it seems like it took longer for either to appear, but the engine was cold and I usually see a pending within the first minute of idling when it's warm.

Sadly, I am back to square one.
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Tiny
Stumasters
Jan 23, 2010.
I think you fixed square one and now you are on square two.A P0171 and a P0174 code is usually related to a fuel delivery issue (lean on both banks). There are many things related to this code, so lets start with the common ones.
Make sure that there are no vacuum leaks down stream of the MAF sensor--especially at the o-ring seal around the throttle body (where the induction tube seals against the TB). Triple check for any vacuum leaks around the intake. Try spraying some carb cleaner around the intake - with the engine running. Observe the idle of the engine -- if it surges or stalls then you have a vacuum leak in that area. Be sure to check at the front and back of the lower intake (because there is silicone sealing these areas-I hope).
Then check the fuel pressure leak down. Hook the fuel pressure gauge up with the engine running - then shut the engine off. The pressure should stay steady for at least 5 minutes. If not, then you may have a leaking pressure regulator or a failing check valve in the feed line of the fuel pump module. Keep us posted.

Tiny
Pear69
Jan 23, 2010.
Well, I can't say I am pleased tonight with my results on the pressures. I purchased a cheaper tester with flow by valve at autozone earlier tonight just so I would have one in my shop. It's the same tester that my Mechanic friend uses, and which gave us such good readings just last week.

Tonight, on this new meter, my results were as follows.

Key on engine off: 53-53lbs at the crossover with about a 60# prime

Idle : 50-52lbs

Throttle dump jumps to 60lbs and back around 48 at letoff.

Flow, using the valve and hose that accompanied the meter sat between 35 and 40lbs when opened fully.

At shutoff, pressure sits at 50 and dropped 5 to 8 lbs in about 3 minutes.

Last week, we measured 54-55 idling, 55 to 60 while driving, although we didn't perform a leak down test.

I am sort of worried that one of us has a bad meter.

Any other ideas. Is there a sensor that might be throwing leans other then a MAF or 02? I will test the pressure again in a day or two and make sure they are the same.

Tiny
Stumasters
Jan 24, 2010.
Do you mean the pressure dropped to 42psi in 3 minutes or do you mean that it dropped to 5 or 8 psi? With the engine running, attatch the gauge and pinch off the return line and read the pressure.
Clean up the fuel pump ground. It corrodes in time. It attatches to the frame crossmember just before the fuel tank. It is the black wire breaking out of the wiring harness in this location. This ground can make your fuel pump pull too much of a voltage drop - thus causing low fuel pressure. Your fuel pressure is not up to spec.

Tiny
Pear69
Jan 24, 2010.
No, I meant once the engine is shut off, the pressure slowly falls about 5-8 pounds in about 3 minutes. I am not sure how long it's supposed to hold that pressure, or how fast it should bleed off, but that's all I noticed there.

I will check the fuel pump ground today, and re-check my pressure to compare numbers. Will post later today with the results.

Tiny
Stumasters
Jan 25, 2010.
Don't forget the return line pinch off test. Pinch the flexible return line off with a pair of vice grips - rap a rag around the line first.
The pressure should hold for at least 5 minutes. This leaking down indicates an issue with the fuel pump or the fuel pressure regulator.

Tiny
Pear69
Jan 25, 2010.

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