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Old 07-17-2007, 12:48 PM
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Engine misses when speed increases

My wifes 97 jimmy has a miss cruising at upper highway speeds. The miss is not constant,it is really noticeable and other times not. I've given the local GM dealership 3 days to figure this out, they could not. There last answer was #2 cylinder has a miss fire code @ 1 MPH and scatters to other cylinders. I installed new plugs,cap and rotor. There are no DTC's for them to go by. I'm really baffled by this one. I have an S10 98, and I'm ready to start swapping parts until I end up with the miss in my truck. Any help would be greatly appreciated. Thank You all!

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Old 07-17-2007, 02:44 PM
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Could be fuel filter. If it is, then gas is getting by at a slow rate, but once you step up the flowrate, it starts to choke up from too much crud in the filter. I had an MGB that drove me nuts for two weeks doing that.

Or, maybe it's the fuel pump. Same symptoms coming from a weak or missing pump.
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Old 07-17-2007, 03:30 PM
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Doc here,

First step is determine if it is fuel or fire..

IF it is fire..

Higher speeds set more load on the module..This makes heat, and the heat will cause the IGT to malfunction..which in turn causes the module to fail under thermal conditions..This is the first step to total failure..

Pull it and have it tested 10 times in a row for heat malfunctions..(any module will work good at 72 air conditioned degrees.) IF a new one is needed, TEST it also before you leave the store..they have a HIGH out of the box fail rate! And once you leave the store, It's your problem...

UNDUE your Tune up...

START with the basics..Compression..dry/wet 125 to 150 all around , + / - 10 % is a good reading..

Vacuum, 17 to 21 In Hg at a warm curb idle With a steady needle is a good reading..IF the needle has a rapid "bounce" that is all over the map..This is indicative of a Bad Valve train..Burned, Stuck, Leaking seals, guides...If you get 1 "bounce that mates with the miss..use a pair of HOME fuse pullers, pull one plug at a time until the miss is constant..That is the offending cylinder. Further inspect it and it's Valve system.

Check your Timing, Static and Full in..10 to 12 static usually is good, and 32 to 34 at about 2500 RPM's with advance (vacuum or EST ) re~Connected/un~Defeated on the latter..

IF all that checks good..

Check the Coil / Coil packs....Ohm out the primary and secondary according to the manual specs..(differs on the type system you have..) A failing coil will miss under load as it starts to go..

DO you have a Crank sensor? Ohm it out, Check the plug for dirty or corroded Contacts..This is also another miss cause as it starts to fail.

Check the Mag or Optic pickup..(Depending on the system you have) Ohm the sensor out as per manual spec.

That should clear most of the fire system..

The first step would be to get the manual for the spec value readings for your engine.

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Old 07-19-2007, 05:59 PM
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reply engine missfire.

I didn't see if you changed the wires, if so make sure they are tight on the plugs. What spark plugs did you install. If you installed the ones with the screw on wire terminal at the top of the plug, these terminals must be tightened with plyers,if left finger tight they will come loose and thus a missfire. Another trick, get an induction type timing light, the type that snaps over the plug wire. Get some duct tape. Tape the button down on the timing light and then tape it to the windsheild. Carefully route the wires(do not get them in the fan)and close the hood. Go drive the car until it missfires, if the light goes out in time with the missfire then its ignition, may be COIL. If your car has a coil pack you may have to try all the plug wires and several road tests.This trick is not fool proof, but it is a good way to check the coil. Another thing you may try is a running compression test. If a standard compression test does not show anything, then try this. Remove the schrader valve in the compression gauge and screw it into the each spark plug hole one at a time and start engine each time, if the gauge bounces down around zero, you may have carbon buidup on the valve face.
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Old 07-23-2007, 08:14 AM
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Engine misses when speed increases

Well I'm ready to start doing alot of ohm's testing. I really appreciate all the advice from you all, docvtte,redneckprofessor and 57 kingrat. I'll keep you posted on my progress. Again Thanks for INPUT.
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Old 07-23-2007, 09:31 AM
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Have you tried replacing the number 2 injector? A injector went bad on mine and it gave the misfire code.
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Old 07-23-2007, 03:01 PM
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engine misses when speed increases

Good to meet you Henry Highrise. When the jimmy was at the dealers they did a top end clean and balance test , thus saying the injectors were fine. The plugs were replaced twice with ac/delco oem plugs. The last time I've driven the truck was 5 days ago. The wife drives round trip about 50 mi. three times a week. The dealership says the plug wires were fine also. The very last thing the shop manager was suggesting that possibly the valves may be the problem? This did not make sense to me since the beginning when it was miss firing all over then narrowed down to #2 cylinder. I've been given some options as to what may be problem areas to check out. Again, Thank You all for all your input... Off to the garage to have some FUN.
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Old 07-23-2007, 03:40 PM
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Quote:
Originally Posted by docvette
Doc here,

UNDUE your Tune up...

START with the basics..Compression..dry/wet 125 to 150 all around , + / - 10 % is a good reading..

Vacuum, 17 to 21 In Hg at a warm curb idle With a steady needle is a good reading..IF the needle has a rapid "bounce" that is all over the map..This is indicative of a Bad Valve train..Burned, Stuck, Leaking seals, guides...If you get 1 "bounce that mates with the miss..use a pair of HOME fuse pullers, pull one plug at a time until the miss is constant..That is the offending cylinder. Further inspect it and it's Valve system.

Check your Timing, Static and Full in..10 to 12 static usually is good, and 32 to 34 at about 2500 RPM's with advance (vacuum or EST ) re~Connected/un~Defeated on the latter..

IF all that checks good..
Fuel Filters

Air Filters

Bad Fuel

Weak pump

Clear Codes and cycle about 10 Ign Cycles.

Dirty Injector

Vacuum leaks

Tiny Crack into a Cylinder from a water jacket..(not big enough to heat, but enough to hamper cylinder performance)

What Do the Plugs look like?

What OTHER (if any) Codes have got?

Valves would be consistent throughout a range AND have more symptoms attached than what you are getting..The dealer just wants the same thing they all do....

YOUR MONEY at a very inflated rated for a shotgun approach, that may not fix the problem.

Doc
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