1997 GMC C1500 Motor races A a high idle-#5cyl. Misfire

Tiny
ABIRCH
  • MEMBER
  • 1997 GMC C1500
  • V8
  • 4WD
  • AUTOMATIC
  • 180,000 MILES
Just bought this truck. In the beginning it ran at 2500 rpm when idling and 1500 rpm in gear. Put it on the reader at Advance. Had a MAP sensor error and an IAC error, also cylinder five was missing. Replaced the MAP sensor and the idle air control, and plugs wires, and pcv valve. Had codes cleared and drove it home (15 miles)

Now it idles at 1800-2000 rpm and 900 rpm in gear. May still be missing but I'm unsure at this point. Checked the vacuum lines. Does seem to be one missing from a small junction (pure vacuum diag. Switch?) On the passenger side of the intake manifold. It is directly behind the alternator, has a blue plug and two vacuum lines. One goes to the can on the drivers side and the other is missing.

If more info is needed let me know. It has the 350 in it.
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Tuesday, July 15th, 2008 AT 7:21 PM

12 Replies

Tiny
MASTERTECHTIM
  • MEMBER
Try spraying carb cleaner at base of intake manifold to see if you have a vacuum leak. Remove oil cap and put hand on top of it and see if you have suction, possible internal intake vacuum leak.
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Tuesday, July 15th, 2008 AT 8:41 PM
Tiny
ABIRCH
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Alright I will try that when I get home from work tomorrow. If there is suction at the oil fill that will indicate what exactly? When I spray the carb cleaner will I see the fluid being sucked in or will I hear the motor begin to flood as you would if you sprayed it directly into the throttle body? If there is a leak I am assuming we're talking about a new intake gasket or worse to repair. Thanks for your time.
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Tuesday, July 15th, 2008 AT 9:53 PM
Tiny
ABIRCH
  • MEMBER
Alright I will try that when I get home from work tomorrow. If there is suction at the oil fill that will indicate what exactly? When I spray the carb cleaner will I see the fluid being sucked in or will I hear the motor begin to flood as you would if you sprayed it directly into the throttle body? If there is a leak I am assuming we're talking about a new intake gasket or worse to repair. Any thoughts on # 5 misfire? Thanks for your time.
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Tuesday, July 15th, 2008 AT 9:54 PM
Tiny
MASTERTECHTIM
  • MEMBER
THE SUCTION ON OIL FILL CAP WOULD INDICATE INTERNAL VACUUM LEAK FROM THE INTAKE GASKET (VERY COMMON) THIS ALSO IF ITS ON THE #5 CYLINDER WILL CAUSE THE MISFIRE ALSO. UNFORTUNATELY ON THIS DESIGNED ENGINE WITH THE INTAKE MANIFOLD, IT HAS TO BE REPLACED ON EVERY ONE OF THEM. THEY HAVE SINCE REDESIGNED THE GASKET TO PREVENT PREMATURE FAILURE.
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Wednesday, July 16th, 2008 AT 6:50 AM
Tiny
ABIRCH
  • MEMBER
Hello, I checked it out. Spraying it had no effect but when I pulled the oil fill cap she stalled. I put the cap back on, started it back up and removed it again slowly. Once it was unscrewed I could feel the suction holding the cap in place. I quickly removed it and place my palm over it and felt a good deal of suction. You were right. Good job. Now I think I need to pull the manifold and check the gasket and the spacer and replace what is broke. Does this sound like the right course of action and can you offer me any insight as to any issues I may encounter, or things to look for or check? I changed the intake manifold gasket on my other chevy truck last winter(1995 chevy 1/2 ton w/305) the gasket blew and I had coolant everywhere. Pulled it apart, drained, flushed and refilled the oil and no more issues. With the 97 being a vortec will there be much differerance? Your advice is appreciated.
Thank you.
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Wednesday, July 16th, 2008 AT 8:01 PM
Tiny
MASTERTECHTIM
  • MEMBER
Was the 95 a vortec? This job is pretty easy. A little trick of the trade is not to remove the upper plenom, just leave it on, you can access the bolts to remove the lower intake with it on. Be very careful to mark and remark the distributor on this engine because you need a scan tool to set the timing if you dont get it back on real close. The fuel lines at the rear of the intake you should leave on and disconnect from the rear where the lines connect. This is definitely your problem, your lucky most people go round and round replacing a whole bunch of parts before its diagnosed as the intake gasket. Let me know how you make out.
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Thursday, July 17th, 2008 AT 5:22 PM
Tiny
ABIRCH
  • MEMBER
I'll begin the teardown this weekend. No my 95 chevy wasn't a vortec. I did have an issue with the distributor on the 95. I marked it but when I put it back together I must have had it off a little because it missed badly. Took a little fine tuning but I got her back in time. No timing light needed that time. What is the best way to mark the distributor on the 97 so I don't make the same mistake twice?
Once more you've been a great help.
Thanks
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Thursday, July 17th, 2008 AT 5:42 PM
Tiny
MASTERTECHTIM
  • MEMBER
Just mark the rotor position now and then mark it when it comes out because the shaft turns when removing. Also just make sure you mark the housing clearly and you will be ok.
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Friday, July 18th, 2008 AT 10:14 AM
Tiny
ABIRCH
  • MEMBER
Thanks for all your help. Once I pulled off the intake I saw where the gaskets had been replaced before. The person who replaced them in the past did not install them properly. The gasket was crushed on the end bolts and I could unscrew the bolts by hand.
Got the distributor back in the right place the 1st time. You were right on the money.
Now I will finish going through the truck.
Brakes, heater blower, broke stud, hole in the muffler, and so on:)

Have a good one.
Alex Birch
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Monday, August 4th, 2008 AT 5:51 AM
Tiny
MASTERTECHTIM
  • MEMBER
Thanks man
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Tuesday, August 5th, 2008 AT 8:51 PM
Tiny
ABIRCH
  • MEMBER
Could I get your insight on one more thing?
Been driving it for a few days and am noticing it begin to hiccup once in awhile. Driving along and it jumps, as if it is having a very fast stalling episode and before you have a chance to worry about it everything is fine. Any ideas?
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Friday, August 8th, 2008 AT 5:38 AM
Tiny
MASTERTECHTIM
  • MEMBER
How did you set the timing? Is the check engine light on? How is the tune up parts like cap rotor and wires?
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Friday, August 8th, 2008 AT 6:03 AM

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