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98 Tahoe Running rough **Update - Any help tonight is greatly appreciated**

J

justinf

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I've got a 98 Tahoe with the vortec 350. When it reaches around 60-70 mph, and shifts into overdrive it will run rough, and the engine shakes, if I drop it into third it will smooth out and be fine, plus if you mess with it it will run smooth after a while.

If you let it run rough long enough, it will throw a code for number six cylinder misfire. I have replaced all of the fuel injectors, the fuel meter and the spark plugs, all of the plugs looked alright, but it had ran for a little while without missing.

The only other thing I have done is switched the ignition module with the one from my 98 pickup, same problem in the tahoe and the pickup runs fine.

Anyone have any other ideas what may be causing this?
 
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when was the last time the transmission was flushed? I have fixed a few of these by flushing the transmission. the issue is varnish that builds up on the locking tourque converter clutch plate. when it goes into od or "locks" you get some slippage. if you will use a good quality flush ( we use bg ) that would be the first thing I would do if it only happens in od.
gabe
 
have you checked the transmission fluid, i know in my jimmy if its a little low and it goes into lock up it will surge and feel like its running rough
 
I don't know if it has ever been flushed. I will try and pick some up to try flushing it, is this something I just drain the fluid, put the flush in and run it some then drain it again and refill with tranny fluid (as well as replace the filter)? I have never done a tranny flush, so just thought I'd ask. I will also double check the fluid level, but I know I have checked it several times since this started and it was fine.
 
ok, what I was suggesting was a bg transmission flush. what happens is the tech puts an specialized cleaner in the fluid. this helps to break down the varnish into a suspension. next the car is run and then hooked to a flush machine the fluid runs out of the transmission through the cooler line into the machine. this fills up one side of a dual sided chamber. as it fills up it forces new fluid out of the other side of the chamber and back into the transmission. after all of this is done ( about 16qts of fluid is used ) and the fluid level is set another additive goes into the transmission. all that to say, I would leave this one up to a tech. I preferr BG wynns also makes a flush and a machine, there are a few differernt ones. you should be able to find a local shop that could do it for 100-130
gabe
 
Sounds like tranny shutter. But since its throwing a #6 misfire, have you checked compresion on #6?
 
icer97 said:
Sounds like tranny shutter. But since its throwing a #6 misfire, have you checked compresion on #6?
Click to expand...

No I have not checked compression on #6 yet, but the more I thought about it over lunch, it didn't really make sense to me that it would be the tranny, and throwing the #6 misfire. I may drive it tonight and let it throw a code again so I can check what it is, I suppose with my luck it could have been the #6 injector going bad, and a problem with the tranny :doah: :(
 
icer97 said:
Sounds like tranny shutter. But since its throwing a #6 misfire, have you checked compresion on #6?
Click to expand...

normally you wont get an intermittant missfire from a compression problem. it will be much more consistant. the code for missfire #6 is the only thing that makes me question my initial thoughts on your transmission. if all the ignition parts are good, eg plugs, wires, i would go get it flushed.

also the only time you feel it is in overdrive? It wont do it going up a hill under med/heavy throttle?

gabe
 
Well, I live in Illinois, so there aren't very many hills, about the steepest hill I come across normally is an interstate overpass :D

But yes it is only in overdrive when it gets up around 60+mph. If it starts to shudder, and I down shift, it will go away.
 
I really belive what you are most likely feeling is trans shudder. what I would do with the missfire is this. if it is always isolated to #6 I would start by swapping plugs, #6 and one of the easy ones to get to. if it stays on #6 i would swap the wire out with one that is similar length. if it stays on #6 post up and I will make you a trouble tree to isolate whats going on.
gabe
 
I'll try a new plug wire on it, I have a new set sitting in the garage, I just put new plugs in it last night. One other thing I forgot to add to the original post is before I put in new injectors, the shudder would go away if I added fuel injecotr cleaner to the gas when I filled it up.
 
i don't think a tranny flush will do any good. I used to work at a tranny shop and we saw that problem alot on the Chevy tranny's. Most cases required a new or rebuilt torque converter
 
fordeater said:
i don't think a tranny flush will do any good. I used to work at a tranny shop and we saw that problem alot on the Chevy tranny's. Most cases required a new or rebuilt torque converter
Click to expand...

I own a repair shop in chamblee, It has been my family buisness since 1959. It is true that occasionally you may need a new torque converter. I have fixed many of these problems with the BG flush.

this is a page off the GM tech website

Torque Converter Clutch (TCC) Shudder
The key to diagnosing the Torque Converter Clutch (TCC) shudder is to note when the shudder happens and under what conditions:

The TCC shudder should only occur during the apply and/or release of the converter clutch, and SELDOM after the TCC plate is fully applied.

While the TCC is Applying or Releasing
If the shudder occurs while the TCC is applying, the problem can be within the transmission or the torque converter. Something is not allowing the clutch to release, or the TCC is trying to release and apply the clutch at the same time. This could be caused by leaking turbine shaft seals, a restricted release orifice, a distorted clutch or a housing surface due to long converter bolts, or defective friction material on the TCC plate.

Shudder Occurs after the TCC has Applied

Important
Once the TCC is applied there is no torque converter (fluid coupling) assistance. Engine or driveline vibrations could be unnoticeable before the TCC engagement.


When the shudder occurs after the TCC has applied, most of the time there is nothing wrong with the transmission. As mentioned above, once the TCC has been applied, it is very unlikely that it will slip. Engine problems may go unnoticed under light throttle and load, but engine problems may become noticeable after TCC apply when going up a hill or accelerating, due to the mechanical coupling between the engine and the transmission.

Inspect the following components in order to avoid misdiagnosis of the TCC shudder and possibly disassembling a transmission and/or replacing a torque converter unnecessarily:

Spark plugs--Inspect for cracks, high resistance or broken insulator.
Plug wires--Look in each end. If there is a red dust (ozone) or a black substance (carbon) present, then the wires are bad. Also look for a white discoloration of the wire indicating arcing during hard acceleration.
Distributor cap and rotor - Look for broken or uncrimped parts.
Coil--Look for black on bottom indication arching while engine is misfiring.
Fuel injector--Filter may be plugged.
Vacuum leak--Engine won't get correct amount of fuel and may run rick or lean, depending on where the leak is located.
EGR valve--Valve may let in too much unburnable exhaust has causes engine to run lean.
MAP/MAF sensor--Like the vacuum leak, the engine won't get the correct amount of fuel for proper engine operation.
Carbon on intake valves--Restricts proper flow or air/fuel mixture into cylinders.
Flat cam--Valves don't open enough to let proper fuel/air mixture into cylinders.
Oxygen sensor--May command engine too rich or too lean for too long.
Fuel pressure--May be too low.
Engine mounts--Vibration of mounts can be multiplied by TCC engagement.
Axle joints--Check for vibration.
TPS--TCC apply and release depends on TPS in many engines. IF TPS is out of specification, TCC may remain applied during the initial engine crowd.
Cylinder balance--Bad piston rings or valves that seal poorly can cause low power in a cylinder.
Fuel contamination--Causes poor engine performance.

--------------------------------------------------------------------------------
Document ID# 10106
1998 Chevrolet Tahoe - 4WD
 
Doesn't the 4L60E lock up in 3rd anyways, which if it were converter shudder, should be evident then as well. At least you'd think.
 
i don't know when it locks up, but every case i have ever seen only had problems in 4th gear.
 
Do the ovbious frist! Replace the #6 plug wire or install a new set. The computer is telling you what is wrong.
 
Thanks for the info, I have some new plug wires that are supposed to go on the pickup, I will try replacing the #6 wire first. The problem I am having seems to be after the TCC has been applied. Other than that I will check out some of the other things listed, it is kind of nice having a second 98 with the vortec 350 so I can swap parts off of it on to the Tahoe before going and buying new parts. :waytogo:

After reading that I wonder if it may be carbon deposits on the valves, there seemed to be quite a bit of carbon on the lower intake when I replaced the fuel injectors.

I have had somebody tell me recently to dump a gallon of pure mineral spirits in to the gas tank when I fill up and it will clear out all the carbon buildup and crap. What do you guys think of this idea. It kind of worries me that it might not be good for the O2 sensors among other things.
 
I don't think it locks up in third, in both the tahoe and the pickup I have, once it shifts into 4th, there is one additional drop in RPMs that I believe is the torque converter locking up, only happens in 4th.
 
lock up is in 4th only.
as to the mineral spirits, If it were an old truck with a carb, this might be ok. but just like you said it has o2 sensors ect. i would do a fuel induction cleaning on it.

1) clean the throttle plate, some kind of fuel system cleaner works best along with a toothbrush. crack the throttle plates open and spray it down with the cleaner. take your toothbrush (old toothbrush) and scrubb the front and back of the throttle plate along with the outer walls where it seals.

2) Im not sure what you would use to do the second part, what we do is mix two separate fuel system cleaners in a pressure bottle. we then pressurize it and it is "fogged" accross the throttle plates for about 30 min. this helps to clean deposits on the backside of the valves.

I will try to find a version that you can buy in an auto parts store

gabe
 
I have tried some fuel system cleaner, and so far no change. Is there a way to check the egr valve? I don't really want to just replace it as it is $140. My manual says to check continuity on pins 2 and 4 and pins 1 and 5 if either one is open, replace valve, however, on the old valve, the new one I bought and the one in my 98 pickup, pins 1 and 5 show a short circuit, while 2 and 4 show open circuit.

this problem is driving me nuts and I am still open to suggestions to try and get this resolved.
 
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