4.0 miss at idle???

juice90 · Mar 29, 2015

New to me 98 mazda B4000 4x4 auto with auto hubs. Seems to have a miss at an idle. New plugs were put in today, wires appear to be ok. Rev it up past 2000 rpm and miss goes away. No codes being thrown.

Haven't found any Vac leaks, YET, and the Vac gauge says late ignition timing, but its just in the red.

Any ideas guys???

RonD · Mar 29, 2015

Is it a steady miss?
i.e. for sure one cylinder is missing not a random miss effecting more than 1 cylinder.

You may have an injector that has a dirty tip, leaning out a cylinder, try a can of Seafoam in the gas tank, see if it cleans it out.
Dirty tip drips fuel instead of spraying it, so air/fuel mix is lean at lower RPMs.

If you have a volt/ohm meter you can test the spark plug wires and coil, while the test doesn't mean they are 100% good it can show if it is 100% bad.
The high voltage/low amp spark is effected by resistance(ohms) which is why there are Specs for resistance in the spark plugs, the wires and the coils.

Next stop compression test, the 3 things a cylinder needs to fire is air/fuel in proper mix, Spark at the right time, and.............compression high enough to allow spark to ignite the air/fuel mix.
Your '98 4.0l OHV has ratio of 9.0:1 compression, at sea level you should see 165psi in each cylinder, subtract 3psi for every 2,000ft above sea level
All spark plugs must be removed and throttle propped open when testing.
The actual psi number is not of any real valve, just because test equipment and parameters vary, you need the results from all 6 cylinders to establish an average and then determine if a cylinder is much lower than the others.

juice90 · Mar 29, 2015

Well, it only seems to be at an idle thru to almost 2000 rpm then it runs fine. I am suspecting the number 6 cyl ( middle drivers side) as that plug was black and dead when I pulled it. It is burning oil, but only at a warm idle. clears up once its being driven, and doesn't smoke when its cold, but pretty sure its valve seals as it sat for 4 years. The previous owner did say it was missing when it was parked and it wasn't burning oil. Injector makes sense, may have to pull it and clean or replace that one.

It runs better each time I tinker with it. IE fresh gas, runs better, new plugs runs better yet, fresh oil, runs a lil better still. Might also just need sum time and miles on it too.

RonD · Mar 29, 2015

That would be #5. Ford/Mazda numbers like this
3 6
2 5
1 4
Front

I would pull #5 plug again and have a look.

Yes, if it sat for a long while then the injectors could/should have varnish residue left inside, Seafoam will clean that out, but doesn't happen over night.

You can change valve guide seals without removing the heads.

And change the PCV Valve, working PCV valve reduces pressure in the valve covers and so lessen the amount of oil that can leak down past a seal.
Lucas makes an oil treatment that swells "rubber" seals, it is a temp fix but a fix none the less.

juice90 · Mar 30, 2015

hmm ok, I thought the middle was number 6.. ok... so where is the PCV valve located on this one anyways?
And I was looking today to see how to pull the injectors out and it does not look like a fun job

RonD · Mar 31, 2015

PCV valve is on the drivers side valve cover.

I would also check your coil firing order, it can be miss connected because of the drivers side numbering
As above, the cylinder numbers are:
3 6
2 5
1 4
Front

But the coil firing order is:
3 4
2 6
1 5
Front

Look here, below the specs, for the 4.0l OHV lay out: http://www.therangerstation.com/tech_library/4_0_Page.shtml

So your confusion may have been from looking at the coil
The #6 wire is in the center of the drivers side of the coil, but that wire should go to the last cylinder on the drivers side of the engine.
So double and triple check that, I get that wrong all the time

juice90 · Jul 5, 2015

An update on the miss. Turns out to be a burned valve on the number 5 cylinder. Ripped the head off and took it in.

Turns out, it was more than a burnt valve. Some one who thought they new what they were doing really messed things up. Seals all shot, 4 guides were toast and all 3 exhaust valves had to be replaced.

That prompted me to yank the other one, which had a poor seat, a guide shot and seals in bad shape.

So 700 bucks later, I have 2 rebuilt heads, back on the truck and it runs fantastic.

RonD · Jul 5, 2015

Thanks for the update and fix, good work :icon_thumby:

4.0 miss at idle???

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