claus290373

hello
my 5.7 tpi has a rough idle sounds like its not firing on all cyl.If i listen to the exhaust it sounds like it backfires.it also do a knocking sound at load and high rpm.

I have check the timing and its fine...
All the wires are at the right cyl...and right order on the cap.
There is a good spark on all plugs..
its fires on all cyl.

Went to my mechanic and he tested the car and said that he could see
that cyl 4 + 6 wasent runing right. if i pull the inj.wire from cyl 4 or 6 the is only a small drop in rpm. if i pull the wire from any other of the inj the car drops a lot in rpm.

All wire,rotor,cap, injectors,tps,iac are new.

Tomorrow i will do a compressions test.
dont know that cam i have but can it be a 4/7 swap cam ??
Or can it be the cylinder head gasket.

hope someone can help.............

Flightoficarus

Just curious, have you checked the fuel pressure yet? I know you said the injectors are new, but it still wouldn't hurt to check it.

Also, even if the spark plugs are newer, what do they look like when you pull them out? They can reveal a lot about running/engine issues

If you have a blown head gasket, that would cause overheating, and coolant to get into the engine oil - does the oil cap on the valve cover look normal when you twist it off and look at the residue?

Also, have you checked for vacuum leaks?

Any service engine light codes?

Definitely inspect the plugs/check fuel pressure/look for vacuum leaks/do a compression test before throwing any more parts at it.

claus290373

Hello
No i have not tested the fuel pressure
but will when i get a tester for it.
But if the fuel pressure is Low why is it
only cyl 4 and 6 that is running poor?
There is no vacuum leaks.
The car doesnt show any code.
The Oil looks fine and just pulled the valvecover
and there is nothing that looks bad.
The last time the car it was running fine with not
sound. The knock sound is what worries me most, Can
that be because of the 4 and 6 cyl not running right?
Thanks Claus

aramirez121

could be your cams maybe got damaged somehow? or one of the valves isnt opening up all the way

paulo57509

Do you know what the mechanic did to determine that #4 and #6 cylinders weren't running right? What method, tools or diagnostic equipment did he use?

This is important as it could point you in the right direction to look (either ignition, fuel or mechanical).

Why didn't the mechanic investigate further?

claus290373

Okay
now i have the result of the compressions test and checked the inj and they are all 16 ohm.have checked the valves and they look fine and opens and close ..

Cyl..1 + 2 + 3 + 5 + 7 +8 are all about 150 psi.
Cyl 4 is only 50 psi
cyl 6 is only 60 psi

The first sparkplug on the pic are cyl 2 and then 4 and 6

now what to do ???

Flightoficarus

Based on those pics, it looks like some oil is getting into the combustion chamber, especially with #4 and #6. Generally, with a low PSI reading as well, this isn't a good sign. Ideally, all 8 cylinders should be within 10% of eachother or so (ex - 150 and 140 PSI would be fine, but 50 is very low)

The question is, what exactly is causing the low compression? The interesting thing here is that the two cylinders in question are adjacent to each other - This can happen when a head gasket is out or going out, but you don't seem to be having any bad head gasket symptoms. Is there any white smoke coming out of your exhaust pipes when the car is at operating temp?

Did you or your mechanic perform the comp test? When you tested the low cylinders, what exactly did you do after that? If you squirt some oil into the spark plug hole and test again, and get a normal reading, the cause is likely a bad piston ring (the worst case scenario).

If you do this, and still get a low reading in those cylinders, you likely have leaking valves, and would need new valve stem seals/a valve job - When you first start the car for the day, do you get a small puff of blue-ish smoke out of the exhaust pipe, and then everything seems normal after that?

claus290373

Ok Will try some Oil in the cyl, but think
its strange if i got to bad pistonrings at
the same time. AS for the headgasket the car havent run much
after this problem started. Really hope its the headgasket it a easy fix
I Will return tomorrow with some more Info.
Thanks for u help

claus290373

I have now tested with oil in the cyl and the compression was 70 psi on both #4 and #6.
Can it be the head gasket between the to cyl thats bad ???
The car isnt smoking and have no water in the oil but if its bad between the to cyl i will not have water in the oil..

Flightoficarus

Well, the good news here is that adding oil to the cylinders didn't cause the compression readings to spike back up by large amounts - That means your rings are probably fine.

In this situation, I would say it could be the head gasket starting to go out between the cylinders, but it could also be leaking valves (Leaking valves are more common on SBC engines, but two adjacent cylinders opens up the possibility of it being a head gasket).

No smoke at all from the tailpipes? Generally, with a valve issue, you will only get a small cloud of smoke right upon a cold start-up, but then nothing after that. I would check this one more time, since your #4 and #6 cylinder spark plugs are getting oil-fouled. Otherwise, it may be the gasket in your case.

claus290373

Gasket is gone between cyl 4-6