Ok my 96 Tahoe has a constant misfire but gets worse under a load… Compression checks out fine just did a complete tune up plugs look fine… vacuumed gauge is steady at 18… And it’s not throwing any codes… I’m pulling my hair out trying to figure it out
Have you checked the fuel pressure against the factory specs? Does it bleed down? If it does, you need to check the fuel pressure regulator. Even if it doesn’t, you may still end up pulling the plenum to check the central fuel injection unit. If you haven’t replaced that recently, it is suspect. They tend to go bad with age. Look for any evidence of leakage or chafing on the lines leading out of the unit to the poppets. Those lines often wear through, leak, and cause a misfire that can cause one to pull their hair out trying to find the cause.
Are you sure there’s a misfire in the engine? Because if the vacuum gauge is holding a steady 18" Hg while the engine is running there’s no misfire occuring. The needle on the vacuum gauge would bounce each time a cylinder misfired.
Tester
See I don’t know… It sounds like a mis or dead cylinder and when I’m climbing a hill you can hear it really good… Plus it feels like I’m not getting all the power the engine has to offer
Oh and lately when I remove my gas cap it is under pressure
Could it be that what you hear is not the engine missing, but instead the engine knocking?
Tester
It’s not knocking
Timing chain!
Tester
When you say complete tune up, and then say plugs. I want to make sure to did the wires as well?? Also I think the 95 still had a cap and rotor, did you replace these?? If so then the fuel system is the likley culprit.
Yes I did cap rotor plugs and wires
Is it possible that you’re hearing an exhaust leak? Perhaps a crack in the exhaust manifold?
OK, have you verified, and then re-verified the fireing order??
Lol yes it’s numbered on the cap
A bad fuel injector would be my choice. I had a similar problem on a 94 GMC Truck. It was a dealer fix.
The vacuum gauge reading really should not have a steady reading of 18". An ignition miss generally causes a quivery needle.
Idle smooth or rough to some degree?
What were the compression readings?
180-190 and the number one at 200 and the idle is fairly smooth if you look at the motor you can see it shutter
I’m going all the way back to mark9207’s initial comment which you haven’t said anything about. Have you checked the fuel pressure? If that looks good then you need to keep digging into the injector system.
If it idles smoothly but misses under load, this is the most likely thing, though you could also be looking at an ignition coil issue.
If you rig the vacuum gauge up so its inside when you drive, watch for it to drop as you get under load. This would indicate an exhaust restriction and would account for a lack of power.
No I haven’t checked the fuel pressure I’m going to do it tomorrow
The compression is good so that’s not an issue; thankfully. Next thing I would look at would be the fuel pressure and checking for an exhaust restriction as mentioned.
If all else checks out good, the ignition module is a strong suspect as well.