99 Tahoe Engine Rough at Cruising

Hi all,
I have been a member for years, but you all have been such a help that I never needed to post. Thanks.

Now I do have a problem that is confusing me. I have a 99 tahoe, it runs rough at crusing speeds on the highway. I replace cap, rotor, plugs, wires... and my code reader says that all the sensors are within specs. The problem has slowly gotten worse. I use to only feel it at about 65 and above, now, it's at any speed. It pops through the exhaust and the truck bucks a little. I have thought maybe O2 sensors, but again everything reads in spec and really don't want to throw the money at 4 O2 sensors if they aren't bad.

Any help? Let me know if you need more info. Thanks in Advance

Hi all,
I have been a member for years, but you all have been such a help that I never needed to post. Thanks.

Now I do have a problem that is confusing me. I have a 99 tahoe, it runs rough at crusing speeds on the highway. I replace cap, rotor, plugs, wires... and my code reader says that all the sensors are within specs. The problem has slowly gotten worse. I use to only feel it at about 65 and above, now, it's at any speed. It pops through the exhaust and the truck bucks a little. I have thought maybe O2 sensors, but again everything reads in spec and really don't want to throw the money at 4 O2 sensors if they aren't bad.

Any help? Let me know if you need more info. Thanks in Advance

No CEL?

You may have a dirty throttle body.

Get some throttle body cleaner (NOT carb cleaner), read the directions for use (do not overload to the point of stalling) and with the truck idling, spray the cleaner into the throttle body.
Be prepared for lots of smoke from your tail pipe.

Yea no light... that's whats killing me about trying to troubleshoot this. it seems ignition, but the only thing really left to replace is the coil... I will try that cleaner... It does run completly fine when idle

You should still check for codes even if the light is not on. Not all codes set the light.

What is the fuel pressure at? The dist gear can wear and cause issues. Is the compression good on all cylinders?

//2000CAYukon

You should still check for codes even if the light is not on. Not all codes set the light.

What is the fuel pressure at? The dist gear can wear and cause issues. Is the compression good on all cylinders?

//2000CAYukon

Checked for codes, none. Fuel pressure is 90 psi and just goes up a little when pressing the gas, but not below 90 ever. Fuel pump is new, fuel filter is new.

Was thinking a compression test, haven't yet, but like I said it generally runs alright except when cruising on the highway. Idles good, hasn't stalled.

90 PSI?? I though 60 - 66 was normal. 90 seems high to me.

Is this a 5.7 or 5.3? If it is a 5.7, has the intake gasket ever been replaced?

//2000CAYukon

have you replaced the air filter? sounds to me like engine timing is off. or else you have a restricted exhaust system problem. exwldr...:sunglasse

Now that you say that, I will check the pressure again... I might have remembered it wrong... because 60 does sound right.

I had exhaust done on the truck back last January, but the problem started about a month ago... It was a cat back system, flowmaster.

The intake hasn't been touched... I was thinking maybe that, is there a way to check it without taking it apart to see if it's leaking.

It's the 5.7

have you replaced the air filter? sounds to me like engine timing is off. or else you have a restricted exhaust system problem. exwldr...:sunglasse

I ran BG 44K thru the fuel lines, and cleaned the air filter... It's a K/N

I ran BG 44K thru the fuel lines, and cleaned the air filter... It's a K/N

Did that fix it? Hope so, good luck.

Unfortantly, no... I am thinking maybe a cam or crank sensor starting to go bad... can't think it's an intake gasket though because I would think it would run rough all the time if that were the case. It idles fine, starts right up, has never stalled.

I don't know, very confused:confused:

Try replacing the Cat(s). I had the same problem with my Blazer. I replaced the Cat and the hesitation and Hitching was gone.

Unfortantly, no... I am thinking maybe a cam or crank sensor starting to go bad... can't think it's an intake gasket though because I would think it would run rough all the time if that were the case. It idles fine, starts right up, has never stalled.

I don't know, very confused:confused:

Wayne,

Unless you already know.........

Indicators for a leaking intake manifold gasket are: engine temp peaking, light brown sludge on the bottom of the oil filler cap and coolant slowly but steadily disappearing from the coolant reservoir.

If you notice the reservoir level dropping, first refill ONLY to the FULL COLD mark and check for any visible leaks.

You can also wipe the inside of the valve cover with your finger and see if there is any sludge.

Intake gasket leaks can either be external (visible around the manifold bolts) or not visible, and coolant will leak into and mix with the engine oil (bad news).

Wayne,

Unless you already know.........

Indicators for a leaking intake manifold gasket are: engine temp peaking, light brown sludge on the bottom of the oil filler cap and coolant slowly but steadily disappearing from the coolant reservoir.

If you notice the reservoir level dropping, first refill ONLY to the FULL COLD mark and check for any visible leaks.

You can also wipe the inside of the valve cover with your finger and see if there is any sludge.

Intake gasket leaks can either be external (visible around the manifold bolts) or not visible, and coolant will leak into and mix with the engine oil (bad news).

Well that makes since... I will check that tonight and get back to you... I'm at a lose and willing to try anything right now to keep from taking it to the dealer as I am kinda tight on money right now.

Thanks for the all the help so far guys, you have given me a lot to think about.

I re-read your original post and I see a reference to "popping' through the exhaust.

Up to this point, none of us has addressed this issue.

By 'popping' do you mean a noise like a small backfire?

When it starts is it constant? At what speed? X mph to X mph ?

You said "any speed", does that mean almost idle too, or when accelerating?

You said you replaced cap, rotor, plugs and wires. Still have the OLD COILS?

Was there one plug a lot worse condition than the others?

The 'popping' noise leads me to think pre-detonation.

Please answer ALL the questions as best you can so we can get a better understanding of the problem.

I re-read your original post and I see a reference to "popping' through the exhaust.

Up to this point, none of us has addressed this issue.

By 'popping' do you mean a noise like a small backfire?

When it starts is it constant? At what speed? X mph to X mph ?

You said "any speed", does that mean almost idle too, or when accelerating?

You said you replaced cap, rotor, plugs and wires. Still have the OLD COILS?

Was there one plug a lot worse condition than the others?

The 'popping' noise leads me to think pre-detonation.

Please answer ALL the questions as best you can so we can get a better understanding of the problem.


I wouldn't really say it's a backfire, it's like the engine is missing. When I have my foot in it, there is no issue, when I let off completely or just in it a little to maintain is when I feel it.

Truck idles fine, starts right up, has never stalled out, plenty of power still. I did not replace the coil... coils? is there more then one?

All plugs looked fine and didn't look like the needed replacing at all, but did it anyways.

Now when I bought the truck a year ago, the guy had all the work done at the dealer, I went to the dealer and printed out the records and nothing looked unusual. Truck has 115,000 on it now. I bought it at 89,000. I do a lot of traveling with it. It threw a code twice when I first bought it, misfire on cylinder 3 and cylinder 5, about a month apart from each other, I bumped up the octane fropm 85 to 87 and that problem has never come back. I am starting to think there is a link somewhere.

Hey guys, thanks for the advice on things, granted I am more confused then ever, but at least I have some things to look at.

I wouldn't really say it's a backfire, it's like the engine is missing. When I have my foot in it, there is no issue, when I let off completely or just in it a little to maintain is when I feel it.

Truck idles fine, starts right up, has never stalled out, plenty of power still. I did not replace the coil... coils? is there more then one?

All plugs looked fine and didn't look like the needed replacing at all, but did it anyways.

Now when I bought the truck a year ago, the guy had all the work done at the dealer, I went to the dealer and printed out the records and nothing looked unusual. Truck has 115,000 on it now. I bought it at 89,000. I do a lot of traveling with it. It threw a code twice when I first bought it, misfire on cylinder 3 and cylinder 5, about a month apart from each other, I bumped up the octane fropm 85 to 87 and that problem has never come back. I am starting to think there is a link somewhere.

Hey guys, thanks for the advice on things, granted I am more confused then ever, but at least I have some things to look at.

More than one coil?

My '02 5.3L has one for each plug.

I don't know if your '99 has them or not.

If the plugs were all the same, (firing) then you likely do not have a bad coil.

You had to raise the octane rate ? You DO have the recommended plugs installed right?

Our intent is not to confuse but to help you understand the problems.

Without making me re-read all those past posts Wayne, do you have a repair manual for your truck? You can buy a Haynes manual at almost any auto supply store for about $20. They are well worth the money.

check the fuel regulator vacuum line for any gas vapors,,,,could be the diapham is split causing too much gas to enter the intake manifold..

this vehicle is a 5.7liter with one coil and has a distributor cap, and rotor...??? .many where built with the 5.3liter and have 1 coil/cylinder...no distributor cap...

More than one coil?

My '02 5.3L has one for each plug.

I don't know if your '99 has them or not.

If the plugs were all the same, (firing) then you likely do not have a bad coil.

You had to raise the octane rate ? You DO have the recommended plugs installed right?

Our intent is not to confuse but to help you understand the problems.

Without making me re-read all those past posts Wayne, do you have a repair manual for your truck? You can buy a Haynes manual at almost any auto supply store for about $20. They are well worth the money.

I put the bosch plats in there, but after reading the posts... no one here likes those... I have always used them and never had problems. I was thinking of just going to the dealer and getting OEM... The gap seems really high though, I am use to .45, but the truck is saying .60? is that right?

Yea the 5.7 that I have only has one coil...

You guys aren't confusing me... the truck is, I just wish it would throw a code and make life easier for me.

Yea I have the books for the truck... I have owned these trucks for a long time and thought I knew their qwirks... I also have a 95 Yukon with 160,000 on it still kickin strong.

check the fuel regulator vacuum line for any gas vapors,,,,could be the diapham is split causing too much gas to enter the intake manifold..

this vehicle is a 5.7liter with one coil and has a distributor cap, and rotor...??? .many where built with the 5.3liter and have 1 coil/cylinder...no distributor cap...


Never thought of that... I will have to look at that. Where is the reg located on this engine?

Never thought of that... I will have to look at that. Where is the reg located on this engine?

I just replaced mine. It's under the upper intake manifold on the injector housing. There's a good thread here: http://www.automotiveforums.com/vbulletin/showthread.php?t=918239

It's from the Suburban forum, if you have any questions I'd be happy to pass on what limited knowledge I've got.

[quote=Waynehead99]I put the bosch plats in there, but after reading the posts... no one here likes those... I have always used them and never had problems. I was thinking of just going to the dealer and getting OEM... The gap seems really high though, I am use to .45, but the truck is saying .60? is that right?

Yea the 5.7 that I have only has one coil...

You guys aren't confusing me... the truck is, I just wish it would throw a code and make life easier for me.

well I have used the bosch +2 plugs and have had no problems....as you have read many have had issues with bosch...I have them in the Lt1 and also the 5.3 vortec..the vortec aluminum heads do cause the plugs to be changed more frequently and this includes the a/c delco plugs....


The LT1 engine has cast iron heads and the plugs last longer as with many miles they still look new..


the gap I believe on a plug that is gap adjustable is .060 in ,,, but check first with your vin # call dealer to get correct plug/gap..numbers..


remember to check wire resistance new wires don't mean good wires..

I hear that the fuel injection setups on Tahoes 96-99 can be faulty. The fuel delivery system is under the intake. It's like $180+ to replace it. The part that distributes the fuel to the series of tubes underneath the intake can wear. Or possibly a bad injector. Just some things I've heard about these vehicles that they're infamous for. Good luck.