1994 454 missing and running on 7

[Clown 32]

History.
It missed every once in awhile, than later I stomped on it and it backfired pretty loud and blew the 3" exhaust apart where it connects to the muffler, now real loud. It runs and starts but it has low power and idles rough and gets bad bad gas mileage, like it got good gas mileage before....lol It doesn't backfire anymore when driving just has low power and idles rough.

I put new plugs wires, cap, rotor, ignition coil on (acdelco). What else can I check for a miss? The tach is jumping when accelerating with low power. I think the #1 cylinder is not firing. I took the plug off and it was dark black it should have a tan color. I checked the plugs for spark and that was good, compression is 120 in #1 cylinder. Somebody told me to replace the dist.? Could it just be out of time? Could it have a broken valve? With 120 psi wouldn't this say that it is ok internally?

Any advice?

1994 454 3/4" ton ext cab long box - 185,000 miles

[MT-2500]

Start with a full/all cylinder compression reading.
Post back readings.

Is no 1 cylinder getting good hot spark to plug?
Ac delco plugs?
Check dist shaft bushing for excessive side play?
Check Base timing.
What is fuel pressure?
Any bad vacuum leaks?
Any check engine lights?

[Clown 32]

Originally Posted by MT-2500

Start with a full/all cylinder compression reading.
Post back readings.



Is no 1 cylinder getting good hot spark to plug?
Ac delco plugs?
Check dist shaft bushing for excessive side play?
Check Base timing.
What is fuel pressure?
Any bad vacuum leaks?
Any check engine lights?

I'll check compression and post back.

Yes, I switched #3 plug with #1
Yes
Ok I will have the shop check this.
Ok I will have the shop check this.
Ok I will have the shop check this.
Ok I will have the shop check this.
I don't believe this truck has a computer in it so no check engine light. Or no check engine light has come on.

[tblake]

I agree a full compression test would be where I would start. However, keep in mind that a rounded off camshaft lobe on the exhaust port will show compression but cause a miss obviously.

How did you determine #1 was a dead cylinder?

[MT-2500]

Originally Posted by Clown 32

I'll check compression and post back.

Yes, I switched #3 plug with #1
Yes
Ok I will have the shop check this.
Ok I will have the shop check this.
Ok I will have the shop check this.
Ok I will have the shop check this.
I don't believe this truck has a computer in it so no check engine light. Or no check engine light has come on.

Unless it is a carberated engine it has a computer.
Does a check engine light come on key on bulb check>

[Clown 32]

Originally Posted by tblake

I agree a full compression test would be where I would start. However, keep in mind that a rounded off camshaft lobe on the exhaust port will show compression but cause a miss obviously.

How did you determine #1 was a dead cylinder?

I took the truck in to a shop last week friday. They worked on it for 20 min and told me the #1 was not firing and only had 120psi. I needed the truck back to drive so I picked it back up. I am taking it back in on Thursday. I just thought I could look at a few things before I take it back in.

[Clown 32]

Originally Posted by MT-2500

Unless it is a carberated engine it has a computer.
Does a check engine light come on key on bulb check>

No check engine light has ever come on with this truck. Ok it must have a computer because it has a throttle body and fuel injection. No carb. I thought 96 was when they put computers into GMC trucks?

[jdmccright]

TBI started being introduced in Chevy Blazers in about 1986 and in C/Ks in 1987. Along with these came the first vestiges of computerized engine controls, eventually encompassing OBD-I around 1993-4. Check for a burned out CEL bulb...replace with LED and never worry again.

[Clown 32]

Originally Posted by jdmccright

TBI started being introduced in Chevy Blazers in about 1986 and in C/Ks in 1987. Along with these came the first vestiges of computerized engine controls, eventually encompassing OBD-I around 1993-4. Check for a burned out CEL bulb...replace with LED and never worry again.

Not sure what TBI is or OBD and CEL. I do know what LED is Light Emitting Diode.

Please explain the others.

[jdmccright]

Sorry, just assumed you knew. Here they are:

TBI - Throttle Body Injection
CEL - Check Engine Light
OBD - On-Board Diagnostics

You are sort of correct. Pre-1996 used an older OBD version, OBD-I, that could monitor some engine functions and sensors. Updated federal regulations in 1996 upgraded this to OBD-II across all vehicles to improve emissions. This involved using many more sensors and a more sophisticated ECM (Engine Control Module, or computer). That's it in a nutshell.

[tblake]

What is odd is that the early OBD2 systems on GM V8's seems like if a cylinder is missing it has a hard time picking up on it and tripping a DTC. Like if a cylinder has low compression or (in my case) plug fouling out, the SES light will not come on. Only if it is a dead missfire like a wire problem will the PCM trip a DTC.

[tblake]

If the shop found #1 had low compression, why didn't they do a leakdown test right there when they had the plug out? Then they could tell you more than just "low compression on cylinder #1".

Be carefull driving it like that as the unburned fuel could over heat and plug up a cat converter.

[Clown 32]

Originally Posted by tblake

If the shop found #1 had low compression, why didn't they do a leakdown test right there when they had the plug out? Then they could tell you more than just "low compression on cylinder #1".

Be carefull driving it like that as the unburned fuel could over heat and plug up a cat converter.

They only worked on my truck for 20min.

[Clown 32]

UPDATE:

I am running on 6 cylinders. Number 3 and 4 only have 70 psi. CRAP

What to do now, shop quoted me $500-$700 just to take apart the motor to see if it is rings or valves. He didn't quote me what else it would cost to do a fix if it is valves or rings......gosh trucks only book worth $2000 - $3000?

Trade it in and go under the credit knife?

What about this truck?

2003 Chevrolet Silverado 3/4 ton 4x4 Extended Cab 55,000 miles v8, Gasoline

Air Conditioning,Dual Power Signal Mirrors,Power Steering,Power Brakes,Power Door Locks,Power Windows,Cassette Player,Alloy Wheels,Gauge Cluster,Clock,Trip Odometer,Tachometer,Tilt Steering Wheel,Cruise Control,Tinted Glass,Interval Wipers,Rear Defroster,Carpeting,Reclining Seats,Cloth Upholstery,Power Drivers Seat,Split Front Bench Seat,Map Lights,Compass,Bed Liner,CD Player,Running Boards,AM/FM Stereo Radio,Rear Seat Shoulder Belts,Anti-Lock Braking System,Center Arm Rest w/Storage,Passengers Front Airbag,Driver Side Air Bag,Passanger Airbag On/Off Switch

$17990

[tblake]

If both cylinders with low compression were on the same bank, I would wonder if a head gasket blew between them. But 3 and 4 are on different sides (or are the cylinders numbered diffeent on a 454?).

The dealer won't give you very much for it with it running the way it is.

I wonder how a shop can go from saying "missfire on #1" to "low compression on #3 and #4". Do you have a compression tester? Would you want to run a compression test on it on your own?