Big Issue With My Suburban.

[CharlieNoxious]

So, it started with misfires at 70 mph. The engine light coming on, the misfire happening once, the engine sputtering, then keeps going like nothing happened, only for some reason the truck seems to lose momentum at about 70mph anyway. Eventually, I use the truck more and more, get the spark plugs done, air filter changed, etcetera.. single misfires keep happening at 60-70mph. Same thing every time. As I keep using the truck for more high speed driving, eventually it seems to stop misfiring. The code clears itself, and it stops shaking so bad at high speeds. In the middle of this process, my exhaust gasket pretty much exploded as I was driving. I haven't gotten that fixed yet, but now the truck sounds like a civic with a fart can which doesn't bother me as much as you'd think.

 

Anyway, just tonight I was driving. I wanted to give her a little hard test and gunned her when I was sure no cops were around. Got up to 70, my RPM kept climbing, and I felt the truck moving faster, and faster. Then eventually I got to 75 without incident, but had to slow down because by now, there was a bridge and a curve coming up. Then once I had passed that, I tried it again.. got up to about 65, and all of the sudden I just feel the momentum stop. The RPM drops to zero, the truck keeps rolling fine but slows down to nearly 45, and keeps going again. My foot was on the gas as hard as my conscience would allow which was almost to the floor, and it just started slowing down. No strange noises, but the truck did seem to start vibrating during the incident but stopped. The engine light did not come on, I was close to home now, and tried it a third time, and I could only get up to just past 65, and it was alright. My temp guage was set perfectly normal, I've never seen this thing get hot yet, but I got home. Parked in my drive way, and all I could smell was just hot metal, I lifted up the hood and there was no heat coming from it that seemed alarming. So please guys, try to help me figure this shit out, because mechanics around here seem to do a lot of bullshitting before finding the issue. The truck's a 99 suburban with 145k miles on it.

[dlewis1111]

Go to advance auto or autozone and have them read the codes in your computer. it's free...... start there

[Bowler807]

Have you checked to see what codes were being thrown?

 

Just a guess here. Since you had an exhaust gasket "explode" and the truck runs better now, it sounds like you have a severely plugged catylic converter. You have so much back pressure in the exhaust system that the engine can't breathe.

[Z45]

Has the fuel filter been changed recently?

 

I had a 99 K2500 burban at work. I was having an occasional high speed miss (no shaking) on and off for about a month. The burb was approaching 90,000 miles so I had the GM dealer look at it. They said it needed new plugs, wires etc. BUT for whatever reason they never replaced the fuel filter. A week later the fuel pump died, had it towed to another GMC dealer to fix and they said the fuel filter was clogged. They cut it open and showed me the dirt in it. After the pump failure, the high speed miss never occurred again.

[nickvrebel]

catalytic converter get my vote. When they fail they don't always set a code. Take a rubber mallet and whack it a few times lightly and see if there is any rattling going on in there, if so it needs replacement, be sure to check both sides, especially the side with the blown exhaust gasket.

[CharlieNoxious]

The code thrown was a cylinder misfire code.

[bronyaur]

Symptoms are similar to what I've experienced with a cracked distributor cap, although typically when that happens it's not intermittent. You said you replaced the plugs, but with a misfire code and the symptoms you have, I would check the wires, cap, and rotor. If those are okay, I'll go along with the other suggestions that have been made.

[rodram]

- intermittent stumble can be caused by a bad set of plug wires

 

- but as suggested here, I would get the cats checked. something was hot, even when the engine was not overheating

 

- and just to rule out a fuel supply (fuel pump and filter) issue, get the fuel pressure checked... if this is a 99 with a 5.7L Vortec then the fuel pressure should be 61 to 66 psi

[CharlieNoxious]

Sorry, I should of been more specific. But the wires, rotor, cap, and distributor were also replaced with the spark plugs. Didn't fix it.