Vortec misfire > 3000 rpm??

davep · October 20, 2003

It seems that our never-ending list of things to fix on our 1998 Suburban is getting longer. My wife had been complaining that the truck had no power and stumbled on hills etc. I figured it was tranny related, because we're quickly approaching the 4L60E recommended replacement interval (100k mi), but decided to do a little more investigation. Here's the symptoms:

1. On slight inclines, when you add a small amount of throttle to keep

speed, it will "jerk" every once in a while... Like the TC lockup disenaged

and re-engaged.

2. Downshifting to 3rd gear on the highway, it will barely make it to 3000 rpm,

and bucks like its misfiring constantly. All the way to the floor -- no more accel.

3. In the driveway in park: I can slowly bring the throttle up to 4k or so, but if I

punch the throttle down quickly, it vibrates a lot (shakes the truck a bit) and

struggles to rev up... If I let off the pedal a little, it evens out and seems to hold

the rpm.

I had just replaced the fuel pump a month prior, because the previous one left us stranded one day.

After witnessing the above behaviour, I replaced: cap, rotor, ign module. And got a fuel pressure tester, which shows that the new pump is functioning fine (~62 psi first shot)

I'm at a loss. Any suggestions? Oh, and during all these interesting test cases, no codes were thrown by the ECM. None!!

I suppose I'm gonna have to take it into the dealership to see what they can find, but I want to make sure I've covered all my bases first.

Thanks for listening!!

-Dave Perry

tmf6902 · October 20, 2003

maybe a bad fuel injector ot two????? Just a thought

tmf

Import Hater · October 20, 2003

3. In the driveway in park: I can slowly bring the throttle up to 4k or so, but if I
punch the throttle down quickly, it vibrates a lot (shakes the truck a bit) and

struggles to rev up... If I let off the pedal a little, it evens out and seems to hold

the rpm.

That rules out the tranny. Check the TPS. Or could be a bad MAP sensor. Or does it use the MAF? I forgot. One of those.

Rob

Wingnut · October 21, 2003

I was going to say check the codes....But you say it's not throwing any...

Now I say, check the SES light itself.

That type of behavior usually signifies a sensor problem, and you might be throwing a code, and the light just doesn't work.

Of course, if you had problems with your fuel delivery system recently....Like tmf6902 said, I would look at your injectors next. You could have a couple of bad ones. I have known quite a few people that have had them go bad on them, and no SES light was thrown when it happened.

Just my .02

davep · October 21, 2003

Thanks for the replies... I have a code reader which is showing no stored codes in the ECM...

I wish I had a scantool to see the O2 sensor readings and see if I'm getting any misfilres logged... I will have to pull the cpi cover off this weekend and take a look. I have this nagging feeling that its been running very rich and has smoked my cats... I'm guessing the O2 sensors are probably not working that well either.

d**n this is gonna cost me!!

davep · October 21, 2003

Just got back in from doing more fuel pressure testing. It drops down to 52 psi while running, down from the static 60 psi with the engine off which is in spec. However, once I turned off the engine, during the 5 minute period following, it droped over 10psi, which leads me to believe that one or more injectors are leaking down... Grr...

Guess I'm gonna pull them this weekend and try to get them tested...

toddgotfish · October 21, 2003

i might have missed something

but i ran accross a similar prob once

if you haven't replaced the plugs go there first

then get you a new set of wires (mine were bad and felt just like a tranny problem

if you know how to check your firing order and wires , do that

those trucks will run with 2 wires crossed easy , just not right

if those don't pan out it sounds like your ign coil may be grounding out

go outside tonite and lift the hood with it running and look for a spark aroung the coil or a wire somewhere grounding out

you should be able to hear it if it is

let us know how its going

Colorado Kid · October 23, 2003

Fuel Filter?

lowflyer · October 23, 2003

The drop in fuel pressure is the key. You are probably leaking fuel within the Intake manifold from the CPI unit. When you pull the upper manifold off, if fuel is pooling on the LH side, the fuel supply line is probably leaking at the injector housing, if fuel is pooling on the right side, its leaking from the fuel pressure regulator diaphram assy. You can enregize the fuel system to check for any obvious leaks as well. Good luck and let us know what you find.

Also, did you check your EGR system? EGR valve actual counts should be at 0 at idle and KOEO.

Jet-X · October 24, 2003

I've had the same exact symptoms on my 97 GMC Sierra (5.7L), twice. It was fouled fuel injectors. Two the first time and three the second. I drove it to the dealer and it would barely do 40mph on the freeway, misfiring the whole way. It had check engine light, at the time. General Motors later issued an extended warranty notice to all CA residents blaming the poor gas quality. It covered the entire fuel system for 100K miles. I sold it shortly after that.

gixxer · October 30, 2003

just gettng back to basics buthave you changed the fuel filter??????

davep · November 3, 2003

Okay, an update: Leaking fuel pressure regulator.

Took, off the upper manifold and the rear was wet with old gas... The front of the manifold was actually fuzzy from the soot caused by the over-rich situation from the EGR system. So I pulled the intake, cleaned it all up, put it all back together with a new regulator, changed the spark plugs (sure enough 8 and 6 cyls were coated with wet soot). Went ahead and put new wires on, and the coil I had sitting around.

Much better... Pulls strongly up to HP peak... I'm very happy!

Thanks for all the advise and pointers...

-Dave

454ss · December 10, 2003

does anyone know if the fuel pressure regulator is the same on the 454. my truck with a 454 is acting the same way but i dont know where the regulator is to check it. thanks

Bluesman123 · January 4, 2004

Dave, you didn't mention (or I missed it) what year your Suburban is. I suspect from what you have posted that it is the older CSFI system. GM has issued a tech notice on these systems (don't recall the number). If you have repeat problems with your CSFI system they are authorized to replace it with a newer MFI system, at no cost to you, up to about 100K miles.

When you took the top off your manifold and cleaned it up, you must have looked carefully at the "octopus". If each leg had a wire running down it to an injector then it is the newer MFI system. If each leg was just a black tube...no wire...then it is the older CSFI system. These leak and carbon up so badly that they will eventually block and burn the valve.

Since I think you are close to the mileage limit on this recall I'm also going to PM this to you.

Good luck,

Bluesman

tclark2020 · December 29, 2006

I am having a similar problem with my '99 Tahoe. When we pull the camper (or trailer) it does fine until it pulls down hard going up hill. The 'service engine light' comes on and stays on for a day or so and then usually clears itself. I have taken it in to my buddies shop and hooked it up to the computer and it is showing a misfire on 3 cylinders. One cylinder has only a few misfires, the next is more, and the third (cylinder 7) is usually outrageous. I have replaced cap, rotor, plugs, wires, fuel filter twice and am still having the problem. Any ideas? Thanks.