Compression test results 3RZ-FE... chasing rough idle

I've been chasing a rough idle for a couple months now, which is starting to get rougher now as I accelerate from stop. Truck is about to hit 180k miles. No smoke, no coolant loss. Rough idle started in cold weather only.

Replaced spark plugs, wires, coils, coolant temp sensor, IAC, PVC valve, cleaned MAF, tested for vacuum leak. Next I was thinking it could be fuel injectors, but wanted to do the compression test first before I went that rout.

I did a two sets of cylinder compression tests todays. First dry, second with oil added. Removed all spark plugs, removed EFI, and killed fuel pump beforehand. Also had throttle fully open.

Here are the results...

Test #1 (dry) listed front to back.
Cylinder 1 - 225
Cylinder 2 - 200
Cylinder 3 - 170
Cylinder 4 - 198

Test #2 (oil added)
Cylinder 1 - 256
Cylinder 2 - 232
Cylinder 3 - 215
Cylinder 4 - 230

Did the test to check for possible blown head gasket between cylinders but now I am even more confused.. especially because cylinder 3 is low, and 1 is so high. Has anyone dealt with this before? I live in Seattle near sea level would that explain the high numbers overall?

 

I'd explore the injector route, maybe by trying some injector cleaner in the fuel tank or removing them and sending them off for a more thorough cleaning [and/or rebuild]. Those compression numbers look plenty good enough I'd think to rule them out as a culprit for the rough idle.

 

Thanks for the response! Good to know about the compression numbers. I did try fuel system cleaner a few weeks ago. It might have run a little smoother during that tank? but then went back to rough again... extra rough a few times maybe the cleaner released some residue from the tank or something. The more I read about dirty injectors the more I think it's better just to pull them out and clean/rebuild like you said.

 

Also while you're going down the fuel route, may be a wise idea to check the fuel filter.

 

Wouldn't hurt to test your Fuel Pressure Regulator, and Throttle Position Sensor. Although FPR is probably an unlikely culprit

 

Yes definitely will replace the fuel filter when I do the injectors.

 

I did check the fuel pressure regulator and it seemed to be in working order (used vacuum pump on the main nozzle and engine stalled). I have not replaced the throttle position sensor but I will definitely add that to the check list. Thanks for your response!!

 

Just did a compression test on my 97 Tacoma 2.7L 3RZ-FE. At first did it with one plug pulled at a time and had cylinder 1 130psi, cylinder 2 130psi, cylinder 3 135psi, cylinder 4 135 psi. I have the fuel pump fuse pulled and had the throttle wide open and let it crank 8 times. I retried with all the plugs plugged pulled and got cylinder 1 185 psi, cylinder 2 185 psi, cylinder 3 180 psi, cylinder 4 190 psi. Not too bad if I do say so myself.

 

So its typical for 2-3 to read worse than 1 and 4 due to the nature of engines and the pressures exerted.

But with 4 bangers and EGR I always suspect excessive carbon build up on the intake valve, this doesnt correlate with the wet compression readings but its just my experience.

The cold start rough idle could be numerous things, but nearly all can be simply attributed to age. Could be carbon, could be poor injector pattern, could be a very faint headgasket leak etc.

I wish these trucks were easier to perform injection services on, but due to the hardlines its a huge pain to connect a proper purge system.

If it was me I'd send it, and do an old fashion seafoam clean. I do it on my older vehicles. The seafoam will hit the valves, not quite the same as the injector clean but it still works.

 

Check your valve clearances…

 

I was about to say this.. the 2.7s are notorious for burning valves.

 

Always a good idea! Usually the valve cover is weeping anyway so even if your in spec your fixing something! I had similar issues with my 2.7 and did new filter and injectors and it seemed to help. Getting that filter out was super fun though.

 

^^^^^right!
Top sides of my hands were swollen from contorting to get to FF

good results after installing
Pretty sure original
Yuck