# 5.7L Running rough when warm. B&B HELP PLEASE!



## heavychevy01 (May 2, 2007)

Hears the deal. Ever since I bought the truck ('97 K2500 5.7L 39kmi currently) I have had slight smell of coolant after the truck is warm. I have never seen any leaks and I always checked the coolant levels and topped when needed and it has never been an issue. In getting ready for the upcoming season I just tuned up (plugs,wires,cap,rotor) and had the oil changed at one of those Valvoline instant oil change jobs. I know, I know, I should have never gone there but it was strictly out of convience. 

Ever since then I have noticed that the truck will run rough both at idle and under acceleration after it has run for a while, say a 15-20 mile trip, feels and sounds like a misfire. When it is cold, it runs fine and it never has an excessive crank time. I rechecked all the previous work I did and found one of the plug wires I bought had a loose connection so I ended up getting ALL new wires and all has been fine, untill today when I used it to get a load of wood. All was fine untill I started it up after unloading the wood and felt the misfire back!


My question is this: I know that these engines are notorious for the intake gaskets leaking. Could this be my problem? Anybody ever hear of this? I know B&B has :laughing: . Any help or insight you guys/girls could give me would be a huge help! 

Thanks!

Jon


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## chris694205 (Dec 7, 2009)

does it throw a service engine light??


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## heavychevy01 (May 2, 2007)

Nope, no check engine light.


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## dheavychevy38 (Nov 19, 2008)

Easy way to check for a leaky intake is to get a coolant pressure tester.


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## 20Silverado05 (Jan 16, 2008)

Hows your oil pressure at cold and when warm . Possibly could have a bad crank bearing since oil thins when its warm.


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## heavychevy01 (May 2, 2007)

dheavychevy38;1090438 said:


> Easy way to check for a leaky intake is to get a coolant pressure tester.


Thats my next step. Although I am almost positive it has a coolant leak, just by the smell and the fact that I have to fill up the overflow bottle about every 2 weeks at this point.



20Silverado05;1090438 said:


> Hows your oil pressure at cold and when warm . Possibly could have a bad crank bearing since oil thins when its warm.


Oil pressure is fine, at least according to the gauge on the dash.


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## BlizzardBeater (Aug 29, 2010)

If your refilling your coolant at a constant rate often, your definately losing it somewhere. If you cant see a leak anywhere, your right the 305's and 350's really liked to go through the stock intake gaskets. Once it warms up nice n hot everything expands including heads and blocks and gaskets which could explaing why it only does it when hot. Does the truck blow white smoke out the tailpipe even when hot? If so, your burning coolant. Coolant in the combustion chamber is one of the quickest ways to kill a motor so this issue deserves top priority from you.


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## 70monte (Feb 5, 2008)

I have a 98 K1500 with the 5.7L and I'm on my third replacement intake gasket in 116,000 miles. The gasket was last changed at about 80,000 miles with what was at the time a revised design gasket. They have since come out with another design with the gasket being metal instead of plastic.

Last month I changed out the Dexcool for the green coolant so we will see what happens.

I would definetely get it looked at before it does damage.

Wayne


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## heavychevy01 (May 2, 2007)

BlizzardBeater;1090714 said:


> If your refilling your coolant at a constant rate often, your definately losing it somewhere. If you cant see a leak anywhere, your right the 305's and 350's really liked to go through the stock intake gaskets. Once it warms up nice n hot everything expands including heads and blocks and gaskets which could explaing why it only does it when hot. Does the truck blow white smoke out the tailpipe even when hot? If so, your burning coolant. Coolant in the combustion chamber is one of the quickest ways to kill a motor so this issue deserves top priority from you.


No smoke when warm, I checked. Would the intake gaskets leaking cause a rough running problem also? I haven't driven the truck since this all happend in fear of doing any more damage. It's going into the shop tomorrow to get a New York State inspection, I will probably have them take a look while it's there. I just dont have the time to replace the intake gaskets if that is in fact the problem.


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## B&B (Nov 4, 2006)

If it's using coolant it likely needs intake gaskets and that isn't something you want to let go as doesn't take much coolant in the crankcase to destroy bearings. But it won't fix your misfire. 

There's many (and very common) issues that cause a misfire in the Vortecs. Just because you're not seeing the MIL illuminated doesn't mean there isn't code or codes stored, so start with scanning it as you'll likely find one or more P03xx codes. May even have it narrowed down to one or more specific cylinders. Then do a fuel pressure check, both running and key off checking for excessive leak down. The FP check is important as it can lead to a faulty FP regulator which will also cause misfires. The scan and FP test covers the most ground for the least amount of work. Muck easier to go from there.


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## heavychevy01 (May 2, 2007)

B&B;1091546 said:


> If it's using coolant it likely needs intake gaskets and that isn't something you want to let go as doesn't take much coolant in the crankcase to destroy bearings. But it won't fix your misfire.
> 
> There's many (and very common) issues that cause a misfire in the Vortecs. Just because you're not seeing the MIL illuminated doesn't mean there isn't code or codes stored, so start with scanning it as you'll likely find one or more P03xx codes. May even have it narrowed down to one or more specific cylinders. Then do a fuel pressure check, both running and key off checking for excessive leak down. The FP check is important as it can lead to a faulty FP regulator which will also cause misfires. The scan and FP test covers the most ground for the least amount of work. Muck easier to go from there.


Thanks B&B and everyone for the comments! I will pass along the info to the mechanic working on the truck. Will post when I get a call back form the shop.


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## heavychevy01 (May 2, 2007)

Well don't i feel like an idot! Got a call from my mechanic telling mt that the rough running is/was caused by a spark plug wire that has been chewed through by some fu**ing rodent. Replaced the wire, problem fixed. And as far as the coolant leak goes, found a small leak in one of the hoses that goes from the expansion tank. 

Couldnt have been happier after getting the phone call this afternoon, going from thinking of spending $1000 to spending less than $100 :bluebounc. Might need to go out and buy myself something nice with all this money i saved.


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## RacingZR (Nov 14, 2009)

Nice to have a good, honest mechanic. Take some of your savings and pick him up a 12 pack of his favorite beer


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## stacks04 (Jan 22, 2008)

Just an information post for everyone with the vortec 5.7l era engines, it is entirely possible to have a misfire/rough running engine from an intake gasket going bad. I have seen the plastic gaskets get "sucked in" so to speak on the intake to head ports and cause all sorts of code like o2 codes, maf codes, ect. Even the newer 5.3 era engines early on had intake problems that resulted in replacing alot of warped plastic intakes. 

To the op, glad you got one of the good mechanics left out there. They are definitely tough to find these days.


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## heavychevy01 (May 2, 2007)

No one needs to tell me how invaluable a good mechanic is. I was a service manager for 7 years in a stealership. I have seen and heard plenty of horror stories. Needless to say, he is worth his weight in gold! He's been my family's and my mechanic for as many years as I can remember. We have always brought our cars to him for tires, oil changes etc... And to RacingZR, he gets hooked up plenty around the holidays. payup


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