1999 tahoe.P0307 Misfire, compression test

[chicagochuck]

OK so I have a 1999/2000 Tahoe 149,000 5.7 vortec

With a cylinder 7 misfire. I have changed the plug wire, spark plug wire for that cylinder and the distributor rotor.

The misfire will be almost immediae. I have a scanner so i can erase the code, and the car will run maybe a couple minutes and the light will come back on.

Problem is this motor was installed last year and the car has only been driven a few times around the block and back.

I checked the fuel pressure and it holds at 50 psi. Then gradually goes down. So I don't think its the spider.

Now do I have to absolutely remove all spark plugs to do the compression test, if its just this cylinder? I know that is recommended.

I have read that for these engines the engine compression should be 160 - 180

So I went ahead and disabled the coil and fuel pump and the 7th cylinder is about 180 psi. So should I proceed to check other cylinders?

I have not changed the distributor cap..maybe that should be next.

 

[bigfootchiro]

Misfires are always a tough one to figure out. First off, the fuel pressure number you have is low, and it should not drop gradually. It should stay fairly stable. 55-60 PSI is recommended, so you may have a faulty spider or FPR. The MPFI upgrade is always recommended. However, that should not be the cause to your misfire, unless that injector is not functioning. I see you mentioned plug wire twice, so I’m assuming you changed the plug itself. With your scanner hooked up and engine running, have you checked all live data for misfires on other cylinders? You can get random misfires on other cylinders that will not throw a code. If so, that could point you in a different direction. Check that and respond back.

 

[chicagochuck]

Yes I changed the plug and switched the wires with other cylinders. same po307. But today I didn't continue to erase the code I just let it run. I tightened a few things like the air box and made sure the hose coming to the intake from the brake booster was more secure. I used a propane tank to see if I could find a intake leak...didn't find anything.


My fuel pressure is 57 psi Key turned engine off
48 PSI While the engine is running.. Is that weird? I will double check all of this tomm. And really check it every 5-10 minutes.....This was before I saw your post.


So today I let the car run for about 15 mins and a po135 code came on. I think I also need to let the car run for a longer time. I just didn't want to damage the converters or anything with the misfire.

I can't really put it on the road because it doesn't have plates or tags, not saying that would help anything. But I prob need to try to get as many monitors to set as possible.


I don't think my scanner has the live data, but a friend might have one that will.

Thanks for your help, I will update..

 

[bigfootchiro]

Fuel pressure should be higher than that while running. Pump may be getting tired. A scanner with live data will help a lot. You can order a Foseal wifi OBD reader on Amazon for $20, and get DashCommand app on your phone for $10, plus GM add on package for I think $10, then you can view practically everything.

 

[chicagochuck]

OK I will check the fuel pressure again and make sure my reading is correct. Would that cause a misfire?
Should I change the o2 sensor I'm going to check the fuses first?


I used a dashcmd app afew years ago with the Bluetooth elm (without the gm add on's), so i have used it before. My Bluetooth elm may still work. I would just need to get the app again

My bro has the scanXL program on his laptop maybe that will help. But I will look into getting the dash app I need something like that anyway.




I saw the Gm add's as a part of the dashcmd app but would I just choose the newest one? They have gm 2004 Gm 2002 etc.. My truck is a 2000.

This is what I saw when I google it. Maybe its more choices once I download it.
© 2013 Palmer Performance Engineering, Inc.
Price
$9.99
In-App Purchases

1. GM 2004$9.99
2. GM 2003$9.99
3. Ford 2006$9.99
4. GM 2005$9.99
5. GM 2002$9.99
6. Ford 2004$9.99
7. Ford 2003$9.99
8. Ford 2005$9.99
9. GM 2001$9.99
10. Ford 2002$9.99