My 350 sounds like it is hitting on 7 cylinders... (75 GMC)

[bobby dee]

Hi Folks,
Brand new to the forum, My truck has me baffled...

Simply put, it sounds like it is only firing on 7 out of 8 cylinders. But it is MUCH more noticeable while I'm driving. It starts VERY easy, Idles perfectly (hardly noticeable at idle). Has me scratching my head.

Here's what I have done...
NEW HEI Cap and Rotor
NEW Plugs
NEW Plug Wires
SET Timing
I have also put my timing light on every single plug wire at idle and it pops the light just fine.

Any ideas?

 

[ChrisPerry]

Did the problem start before or after all the work?

If before, it can be a vacuum leak, valve adjustment, flattened cam, mis-adjusted carb, bad coil, poor connection not feeding enough current.

If after, may be a bad new part or incorrect firing order.

 

[bobby dee]

Before.

Remember, it idles PERFECTLY.

 

[Bent77]

After double checking the firing order...

I’d check and make sure each cylinder is getting spark.
I’ve seen a lot of trucks this vintage lose a cam lobe with no idle symptoms

 

[bobby dee]

Timing light on each wire... flashing with each spark?

 

[Bent77]

I’d use an extra plug to put in the wire and ground it to the block to see if it sparks.

Do you have a vacuum gauge?

 

[ChrisPerry]

I’ve seen a lot of trucks this vintage lose a cam lobe with no idle symptoms

QFT......

Vacuum gauge is a good tool for this. If the gauge fluctuates rapidly it's a sign of cam or valve issues.

 

[tRustyK5]

Timing light on each wire... flashing with each spark?

That only tells you juice is getting sent down the wire, it does nothing to tell you about spark quality or anything else really. It doesn't even confirm if the firing order is right.

An IR temp gun on the exhaust manifold at each cylinder will help you find a dead(ish) hole if you have a cold area. This works best on a cold engine that's only been running a few minutes.

 

[blazer74]

Idles perfectly (hardly noticeable at idle).

Doesn’t go together, one or the other

 

[bobby dee]

An IR temp gun on the exhaust manifold at each cylinder will help you find a dead(ish) hole if you have a cold area. This works best on a cold engine that's only been running a few minutes.

Will this still work if it is just a high rpm OR under load issue? {will the problem cylinder heat things up less?}

I did a similar test to this a while back except only with water-- no noticeable difference.

 

[bobby dee]

Doesn’t go together, one or the other

Fair enough. Change that to it idles nicely.

I will try to post a video (youtube) tomorrow to give folks an idea of what things sound like, will see how it does audio-wise

 

[bobby dee]

QFT......

Vacuum gauge is a good tool for this. If the gauge fluctuates rapidly it's a sign of cam or valve issues.

Vacuum guage on the vacuum line, I presume... I think they "free rent" those down at my local parts store.

 

[ChrisPerry]

Yes. Put it on a full manifold vacuum port. Vacuum should be steady, if the needle bounces while holding a steady rpm there are issues.

 

[bobby dee]

Yes. Put it on a full manifold vacuum port. Vacuum should be steady, if the needle bounces while holding a steady rpm there are issues.

all righty, will give that a go.

 

[rampage]

I did a similar test to this a while back except only with water-- no noticeable difference.

Ummm, yeah.
Don’t do that again unless you want to risk having cracked manifolds.

 

[blazer74]

Is this a high mileage engine?
Did the problem just happen at once?

 

[bobby dee]

Is this a high mileage engine?
Did the problem just happen at once?

Well, its hard to say... either 75k or 175k

It just starts up so beautifully... sigh.
I have another motor for it, I just wanted to drive it thru the winter first... since I have never swapped a motor and dont want to get caught in the weather... (no garage that fits the truck)

 

[RubberFloorMat]

I had a similar issue. The previous idiot had dropped the distributor or whatever when installing the new crate engine and the distributor was frigging bent. Rotor was Was making contact on one side of the cap and not getting close enough on the other for proper spark jump. Also was worn out and would constantly vary between 4* of timing. I'd check fuel and spark first. Those are are your basics anyway.

 

[blazer74]

Fuel filter changed?

I had a quadrajet plug up on one side of the main circuit. Would idle fine but off idle it acted just like it was my missing on a 81 Scottsdale.

Worn cam lobe can cause issues as well as a bent pushrod , cracked rocker arm, weak lifter. Rocker stud pulled out some.

Any abnormal noises from the valve train?
Tested with a vacuum gauge as mentioned?

Check for full 12 volts at the distributor .

If you can’t isolate the problem might need to pull the valve covers for a look static and running. Makes a mess but might see a problem.

 

[chulisohombre]

Have you done a compression test. That will tell you if the rings are still good. Brand new engine will be roughly 150 psi. Anything less than say 110 on a cylinder and it won't fire correct. After you test them all on your engine, anything more than 20 psi from your highest compression tested you have a problem.