1992 ford straight 6

[tri again]

annoying miss at idle, runs great
10-12 mpg?

any suggestions?

[Thorpe]

Bad injector?

[dougspcs]

Assume you have checked plugs, compression, wires..which cylinder? Injectors not usual for the 4.9 unless it has 160,000 plus. Pull wires one by one while idling to see which doesn't change the engine lump...

If not ignition or engine compression, spray quick start around the base of the intake manifold and see if engine changes..if it does pull the intake/exhaust manifolds and replace gaskets. Beware..studs will break alot on this engine.

[tri again]

now why did I not think about pulling wires one at a time.

and yes, new cap,plugs,wires.

sometimes it idles per purr perfectly
esp stopped in gear (automatic) but it never makes sense, hot or cold,
nothing matters.

seems worse with the egr vacuum line hooked up,
off and plugged seems way better.
Many say to add a restrictor plate to the egr valve, thin sheet of something with a smaller hole in it could fix it.

Maybe a general vacuum test? since those plastic lines run everywhere and get brittle.

Thanks you guys

[just ben]

vacuum leaks cause a high idle. do acompression test on all cylinders chances are one will be way off.then I would do a leak down test on it to see where its losing compression.

[dougspcs]

There is a specific test for the EGR operation using a vacuum pump..I don't have the numbers. I have been out of the Ford shop for about 10 years..

If you thinks it EGR remove it..and have it tested. If sticking then could fully explain intermittant idle issue..EGR should never be open at idle, if it's sticking open uncontrolled exhaust recirculating into the intake will cause lean engine condition..and a miss!

[tri again]

There is a specific test for the EGR operation using a vacuum pump..I don't have the numbers. I have been out of the Ford shop for about 10 years..

If you thinks it EGR remove it..and have it tested. If sticking then could fully explain intermittant idle issue..EGR should never be open at idle, if it's sticking open uncontrolled exhaust recirculating into the intake will cause lean engine condition..and a miss!

That makes perfect sense.
I guess even with the egr vac line disconnected, it could still flutter
to odd positions.
It seems to move freely when I apply and release vacuum.
I wonder if I could just plug it off somehow just to see.
Then the egr postion sensor might freak out too.

We have no emission regs here yet so I may try that next.

Thanks

[Rollingmenace]

you could eliminate the egr... but then you will get a check engine light most likely... this can be fixed by installing a resistor in the line and fooling the ecu but i cant remember exactly what size right off the top of my head..... have you pulled codes off the ecu yet that could give you an idea of which way to go..

[tri again]

no check engine light but some say it can still be holding codes
without the light on.

[dougspcs]

It should unless you disconnected the battery..but that may confused the issue.

Since the monitoring the emission system and the faults you are having are mechanical. An engine miss could set an O2 sensor code, but then you start chasing that phantom when the O2 being out or range was the result of the symptom, not the cause.

I'd keep going on the path you are on..you may have set half those faults running the engine and disconnecting hoses.