Constant misfire cyl 1

I recnelty bought a 98 GMC seira for a great price knowing it needed engine work. It has a constant misfire on cylinder one and it causes the whole truck to shake constantly no matter whether idel or acceleration or slowing down. At first I was thinking a burnt exhaust valve. I ran a compression check (the engine was cold though) and I heard a cold compression check means nothing. But in doing so I found that cylinder 1 has the highest compression out of all. I plan on another compression test tomorrow when engine is warm. But until then I was thinking maybe a bad injector on cylinder one? It is a “spider injection system” and I was wondering if it was possible for one injector to be clogged or not working and the rest be fine. I have seen where one will get hung up and cause hydrolock. Anybody else have any ideas before I start tearing into it? Much appreciated!!

I would first check for consistent spark on #1 with an inline spark tester to rule out something simple like a bad spark plug wire. Also check the spark plug carefully for anything that could cause a misfire, eg cracked insulator or particulate lodged in the gap.

Wire plug and button all look brand new. he assured me he replaced them all before he sold to me.

That means nothing. The fact that he recently replaced all the wires could in fact be the reason for the misfire.
Check to be sure the #1 plug wire is seated firmly in the distributor cap. Sometimes aftermarket wires don’t fit as well as OEM, and a poor connection there can cause a constant misfire.

Do the easy, obvious tests first before tearing into the fuel injection. An inline spark tester costs $10 and takes 10 minutes to run the test.

What MIL code is it showing? If you don’t know, find a parts store that will read it for you for free.

Misfire cylinder one. Can’t remember code number exactly. Have scanner of my own

Assuming it is getting spark any other ideas? Will go back and double check all tomorrow to be sure since I have to warm engine for another compression test

Check for spark with an inexpensive tester. Replace the injector. Still no luck? Pull the valve cover and see if the valves on #1 are moving normally…This is a push-rod small block Chevy engine right?

Yes it is. Wasn’t sure if it was possible to just replace 1 injector on this type fuel system. I’ve only seen the whole injection system for sale online and it isn’t cheap.

Then just pull the valve cover and check valve movement. Sometimes a cam-lobe and lifter fail which will kill the cylinder. It will still have very good compression if it’s the exhaust valve that has the problem…

A cold compression check is perfectly valid and anyone who told you that it isn’t is deaddamnwrong.
Trust me on this.

So what kind of compression numbers are you getting?

If the compression (even if tested cold) is in the ballpark of the other cylinders, in this case even a bit higher, then I doubt compression is the problem. I think you can safely discount compression as the problem.

Step back a little maybe, put this in perspective. It has to be something pretty simple. A misfire is like a firecracker that doesn’t pop. It’s either the fuse isn’t getting lit (spark), they forgot to put powder in it (gas), or it wasn’t pressed together tightly enough (compression). It has to be simple as all that.

And you’ve already proved it isn’t the last one, compression. So it pretty much has to be either the spark or the fuel. As posted above, eliminating the spark unknown is probably the next step. And if you prove that cylinder is getting spark, then you know it’s a fuel issue. At that point you’re almost done.

You say you have a scanner. What are you using? A competent scan tool will give you all the information you need to determine the cause of the misfire.

I needed that George. so I’m thinking it could be a injector. Because there is no backfire ever. Which means if fuel was getting to cylinder and not being ignited then it would cause backfire from hitting the hot exhaust. Right? I’m going to pull valve covers also to check valve movement since I’m gonna be right there at it since I have to pull intake to get to injectors.

My scan tool only reads codes and the only code is p0301. Misfire cyl 1 asemaster

Ok4450 my compression test resulted in
Cylinder 1 90 psi
Cylinder 3 75 psi
Cylinder 5 80 psi
Cylinder 7 70 psi


Those are very low compression numbers, even for a 17 year old vehicle with probably lots of miles. Even considering the engine is cold, those numbers are not good

Do a wet compression test next. If the numbers go up dramatically, you’ve got bottom end problems.

If the numbers barely go up, or don’t change, you’ve got valvetrain problems.

Until you’ve got good compression, it’s pointless . . . in my opinion . . . to worry about the fuel injection system

What engine do you have?

It’s either the 4.3 V6 or the 5.7 V8 . . . since the 7.4 V8 had a more conventional type of fuel injection system, and you mentioned the spider

No scan tool can diagnose a worn cam/lifter…Those compression numbers are poor…Did you have the throttle wide open when you performed the test?? (It needs to be held open) How many miles on this vehicle?

Sorry forgot the 1 in front of all those. All in the hundreds believe the truck wouldn’t run with that low of compression lol

Yes throttle to the floor. 244000 miles. And compression was 190 175 and so forth. Forgot to add the ones.