Questions on Misfire for 98 Burb with 7.4l

[Frogprince]

Evening,

I think my mechanic is either not interested in working on this vehicle anymore or is trying to screw me out of a few thousand dollars.

Last month on the way back from the coast lightly loaded we started to have a misfire and rough idle on the bus. It was missing the heatshield on Plug 1 and cut the boot. Not the first time this has happened (broken exhaust stud in the head).

Took to my mechanic and asked him to do a belt, oil change, fix the rough idle, and fix the steering issue. He found a bad control arm, replaced the wires, changed the oil and gave it back. It drove ok for about 5 minutes and started loosing cylinder 3. Lost AC due to the complete lost cylinder.

So I took it back. He diagnosed it with a Cap and Rotor and replaced them, and sent it on the way. I drove it around the block and still had a bad misfire on Cylinder 3 (codes cleared, came right back) so I took back. This last time he said it is an injector but would run a compression test.

Come today he is telling my my valve is gone and I need a new head. Tune of 1.2k. Now I know how to do a compression test so I declined the repair and took it home. I am fairly certain he didn't pull the plugs. I ran the test with no plugs and no oil. Cylinder 1 165 and Cylinder 3 155. He told me I was at 130 and 60 on the phone then in person it was 145 and 70.

So I am looking at new plugs, and a fuel pressure regulator (Guessing due to the strong fuel smell coming out of the Air Cleaner when sitting.

Question is should I do a intake clean with some Seafoam and try to clean the injectors? Or thoughts?

Another is should I challenge the last repair bill since he never fixed the issue and lied to me about the troubleshooting?

 

[SUBURBAN5]

If it was my truck I would challenge him on the repairs because it kinda sounded like a guessing game. Second I would explained he never fixed the issue and is his fault for not testing further. Lot of lazy people out there who just wanna fix the obvious without putting the time for investigating. Different field but if you were my customer and blew a gasket on my performance like that as the tech I would of ate it cause it's my fault for not double checking. Also do not let this guy fix your truck. Doesn't sound like a good mechanic to me

 

[SUBURBAN5]

Wish I could help with the 7.4 but my knowledge is mostly on the 5.7. I'm sure somebody will chime in though. I've heard there's some common issues with the 7.4

 

[Schurkey]

Put a scan tool on it, look at the data stream.

What is the fuel pressure? Does it hold pressure (No leaking injector, no leaking regulator?)

Why does the A/C quit because of a misfire? Misfire will disable the torque converter clutch, but I don't think it disables the A/C.

What does the spark plug look like?

 

[88monteSS]

The injectors on these motors are notorious for leaking, so I would be suspecting them for the fuel smell. At the least I would replace them with the yellow tops from 5-0 motorsports or consider upgrading to slightly bigger ones. One of these days I'll remember to find the injector chart someone made that gives part numbers of ones that'll fit & plug in.

Sent from my XT1254 using Tapatalk

 

[Frogprince]

Put a scan tool on it, look at the data stream.

What is the fuel pressure? Does it hold pressure (No leaking injector, no leaking regulator?)

Why does the A/C quit because of a misfire? Misfire will disable the torque converter clutch, but I don't think it disables the A/C.

What does the spark plug look like?

I am going to go get a fuel pressure tester on loan tomorrow. This was one of the tests I asked to be completed due to the hot start issue the vehicle has. I am guessing it is bad due to pulling the Air Cleaner and it has a strong smell of fuel.

What he told me was that it was a dead cylinder and it was probably preventing the AC from turning on due to the issue. When I got it back after the Cap and Rotor I had AC again. This mechanic right after we purchased it replaced the Compressor and parts due to a cracked housing. AC Still working today with the misfire that we have.

The plug isn't wet with fuel. They are two years old but normal burn pattern on them.

My Scan Tool is el cheapo bluetooth one that doesn't show all the data in the app. Need to find a better app.

 

[Frogprince]

I looked up the 5-0 Motorsports injectors.

While this doesn't sound horrible to accomplish the reality is till the fires are over here in the PNW working out side is almost out of the question.

Currently we are getting a few inches of ash falling everyday.

 

[Frogprince]

Just an update.

Since I had all the plugs pulled. I replaced them all. Ran a can of Seafoam thru the intake. Lots of white smoke and took her out and tried an Italian tune up.

She has power again. She runs great under load. Doesn't feel like she is missing at all. At idle she still rough at idles at 600-630 and I get the Misfire on 3 again.

Tomorrow I intend to swap out the plug wire on 3. I have a spare set from when one broke right after I got it. I had to replace 7 on it since it came apart when I went to remove the plug.

Is there a test I can run to rule out the Injectors?

 

[Mangonesailor]

Is there a test I can run to rule out the Injectors?

About the only solid test you can do on those injectors to verify no leaks is to pull the upper intake, clean up the tips of the injectors, put clean paper towels underneath, and then force the fuel pump to run by putting a jumper wire where the relay would turn it on.

Run the fuel pump for about 5-10min and look for fuel on the paper towels. If they leak, upgrade to the Bosch yellow-tops everyone recommend from 5-0 Motorsports (I did, fuel economy went up on mine).

You can also use an infrared gun to, quickly, check the temps coming out of each cylinder. If cylinder #3 is hotter than most it's running lean. You could move the injectors around (if you're REALLY not wanting to throw parts at it) and see if the issue moves.

Something to try is pulling the crank position sensor and making sure the tip of that thing is clean up any metal sludge buildup.

Something to think about is your dizzy drive gear. After I installed my 0411 it picked up misfires everywhere, but ONLY at idle. It ended up that half the dizzy drive gear had knife-edged and was causing this. Currently the best replacement for this IMHO is to get the Duralast Gold dizzy from Auto zone and use it. Brass terminals, aluminum body (which is also painted, comes with a gear, and lifetime warranty. If the cap ***** up you can get a free one... but you have to pull the dizzy.

What app and Bluetooth reader do you use? I have a OBDlink MX and use Torque pro OR I sometimes use Dash command to monitor data. Dash command will show you crank/cam offset btw.

It could also be an intake gasket leak right there. Are you losing any coolant?

How many miles on it?

 

[Frogprince]

About the only solid test you can do on those injectors to verify no leaks is to pull the upper intake, clean up the tips of the injectors, put clean paper towels underneath, and then force the fuel pump to run by putting a jumper wire where the relay would turn it on.

Run the fuel pump for about 5-10min and look for fuel on the paper towels. If they leak, upgrade to the Bosch yellow-tops everyone recommend from 5-0 Motorsports (I did, fuel economy went up on mine).

You can also use an infrared gun to, quickly, check the temps coming out of each cylinder. If cylinder #3 is hotter than most it's running lean. You could move the injectors around (if you're REALLY not wanting to throw parts at it) and see if the issue moves.

Something to try is pulling the crank position sensor and making sure the tip of that thing is clean up any metal sludge buildup.

Something to think about is your dizzy drive gear. After I installed my 0411 it picked up misfires everywhere, but ONLY at idle. It ended up that half the dizzy drive gear had knife-edged and was causing this. Currently the best replacement for this IMHO is to get the Duralast Gold dizzy from Auto zone and use it. Brass terminals, aluminum body (which is also painted, comes with a gear, and lifetime warranty. If the cap ***** up you can get a free one... but you have to pull the dizzy.

What app and Bluetooth reader do you use? I have a OBDlink MX and use Torque pro OR I sometimes use Dash command to monitor data. Dash command will show you crank/cam offset btw.

It could also be an intake gasket leak right there. Are you losing any coolant?

How many miles on it?

Mileage is 225K.

I use Torque Pro for the app. The OBD Reader is a BAFX Products 34t5

She does leak antifreeze. I know the lower manifold has been leaking off and on. Normally if the truck is only ran for short amount of time she weeps out the front. If I run it for a longer period of time she seems to be okay.

I was looking at 5-0 Motorsports and was planning on calling them. See if they would recommend the new ones or the refurbished ones. Both are Bosch.

I read a test for the dizzy. Pull the cap and see if the Rotor has any side to side play. If it does then it is a sign that it needs a new distributor. I may get this test done today if I feel up to going outside in the smoke and ash.