55Hdtp

I have a 1999 GMC Sierra 1500 4wd with the 5.3 Vortec V8. The engine was replaced last year with a 2000 model 5.3. After it was put in, it had a misfire under hard acceleration, but that was the only time. Now it misfires all the time even at idle. It sounds like it’s running on three or four cylinders when accelerating. I’ve taken it to three different shops, and one of those was the dealership where the truck was originally purchased.

The first shop couldn’t figure anything out, so he sent it to another. That shop drained the fuel tank because they said the fuel looked weird, but they said it wasn’t water. They also said the #7 injector was bad, so they swapped the #5 injector to #7 and put the new injector in #5. When I picked the truck up it was still idling rough and misfiring. Nothing had changed. I then took it to the dealership. They said my fuel pressure was low and that I needed a new fuel pump, but they wanted $900 for parts and labor. I bought a brand new AC Delco fuel pump assembly and installed it myself and changed the fuel filter. Nothing changed. I bought a code scanner and found a P0135 o2 sensor code. I replaced the o2 sensor and no change. I ran another scan and found a P0336 crankshaft position sensor code. I took it back to the second shop because he said he would change the crankshaft position sensor for free since I paid $500 and it wasn’t fixed. He changed it while I watched and did a crank relearn on it using his scanner and nothing changed. I let him keep it for a couple more days. He spent 16 shop hours and he and his whole crew couldn’t figure it out. Then, I replaced the ECM and the battery and still no change.

So here is a list of everything that has been tried so far in chronological order:

K&N air filter cleaned
Mass air flow sensor cleaned
New spark plugs
New spark plug wires
Injector cleaner added to fuel
EGR valve cleaned
Throttle body removed and cleaned thoroughly
Sensors in throttle body cleaned
Bank 1 coil packs (all 4) changed
Fuel tank drained
Fresh gas put in with water remover additive added
Oil and filter changed
Top end lubricant added to oil in case valvetrain was sticking
Checked for vacuum leaks by spraying carb. cleaner all around intake and vacuum lines
In-tank fuel pump replaced
Fuel filter replaced
Oxygen sensor bank 1 sensor 1 replaced
Crankshaft position sensor replaced
Crank relearn procedure performed
Compression test on cylinder 7 showed 145 psi
Leak down test performed on cylinder 7, and there were no problems found
Crankshaft position sensor and camshaft position sensor wiring was tested for continuity and voltage
Engine to ground and battery negative to ground were checked
ECM replaced
Battery replaced


Today I ran a full diagnostic scan using my Autel Maxidiag Elite MD802 code scanner. These are the codes that were found.

P0300 Engine Misfire Detected
P0307 Cylinder 7 Misfire
P0336 Crankshaft Position (CKP) Senor Performance
P0452 Fuel Tank Pressure (FTP) Sensor Circuit Low Voltage (Today was the first time it has ever thrown this code)
P0135 HO2S Heater Performance Bank 1 Sensor 1


Does anyone have any idea what could be causing this misfire? At this point, I completely baffled , and I’m about ready to just sell the truck and get what I can for it. But I would much rather just fix it and keep it if it’s at all possible.

1FastBrick

iTrader: (17)

What about the fuel pressure regulator on the rail? Was that ever checked?

Is it losing any coolant? Have you checked the cooling system by pressurising it with the engine off?

How about oil consumption? Are you seeing excessive amounts of oil in the Throttle body or intake?

55Hdtp

The dealership checked my fuel pressure at the rail and said it was low and that I needed a fuel pump. It hasn't been checked since the new fuel pump has been installed though. I thought maybe the fuel pressure regulator was bad, so I researched how to check it. What I read, was if you pull the vacuum line off the regulator and fuel comes out then it's bad. No fuel came out of mine, so I assume it's good.

It's not leaking any coolant. No, I haven't done any pressure checks on it. How do you do this?

It's not using oil, and there is no excessive oil in the throttle body or intake.

1FastBrick

iTrader: (17)

You can't check a fuel pressure regulator properly without a fuel pressure gauge. They don't always spew Fuel out when there bad.


Were are you located?

1FastBrick

iTrader: (17)

Try swapping the spark plug on cylinder Number 7 with Cylinder number 3.

While it's out check the Gap.

BTW, what type of plugs were used?

55Hdtp

I live in Oklahoma City.

I swapped the #7 plug with #1 about a week ago, and there was no change. I'll have to get a feeler gauge tomorrow. I recently moved into an apartment, and a lot of my tools are in a storage unit 30 miles away. I think they are NGK Iridiums, but I'll check tomorrow when I pull them out to check the gaps.

TIM Z

iTrader: (12)

Since it didn't happen till after the engine swap check for pinched wires or pinched fuel lines , might be getting enough fuel to cruise / idle but not enough flow to support power enrichment .

Like Joeseph said you need to get a fuel pressure gauge on it.

55Hdtp

Sorry about the extremely slow response. First, the weather got bad. Then, I was in the process of moving to Texas.

The spark plugs are AC Delco 41-110 Iridiums. They are gapped to .060".

I also just got another code. It is P0453 Fuel Tank Pressure (FTP) Sensor Circuit High Voltage. I also found that my fuel line was leaking at the threaded connection of the fuel filter on the "out" side. I fixed that and tried another fuel filter while I was at it, and still no change.

One thing I did notice is that when it was in the 20's and 30's outside, the truck seemed to run better. I don't know if it was just coincidence or not though.

1FastBrick

iTrader: (17)

The factory plug gap changed from .060 to .040 when the original plugs were discontinued.

AC Delco 41-110 Iridiums require a .040 gap

55Hdtp

I'll re-gap them to .040" then. I still need to get a fuel pressure gauge on it and see what that is reading. I don't think it's the plugs causing the problem though. The engine sometimes runs so bad the whole truck shakes like a wheel is about to fall off and other times the miss is barely noticeable unless you are accelerating hard. If it was plugs, it should more consistent. Some cylinders have a few misfires, some have none, and number 7 has thousands. Cylinder 7 has some kind of gremlin that I just can't locate. I'm contemplating pulling out the whole engine wiring harness to check it over and test resistance on each individual wire.