M10 Misfire Mystery

[yesterdaylight]

So after a very hard run on Skyline a few weeks ago (max revs to 7500) I have had a misfire on cylinder number 4 (when I remove the plug wire from cyl 4 is still runs poorly, when I remove the wire from all others it runs even worse). Here is what I have checked:

1. Checked Rocker Eccentrics - properly adjusted

2. Replaced Cap and Rotors - have strong spark to cyl # 4

3. Checked Ignition Timing - All good

4. Checked for vacuum leaks - seems ok

Symptoms: Rough idle, rough up to about 3000rpm, smooths out at higher RPM's but a distinct lack of power. Seems like I've got a 3cyl. Next step is to pull the head and see if I have a burnt up valve or broken a ring, however I dont seem to be burning any more oil that usual. Any other advice??

Ian

PS I just moved from upstate NY to the SF Bay area (south bay) for work and of course brought the 02! looking forward to getting together with some local 02 enthusiasts if work ever allows me some time...!

[Mike87]

You might check to see if you have a vacuum leak (read: failed intake gasket) causing your misfire. Also a bad plug can be a cause. Just things to check before you pull the head.

[fxtbass]

COMPRESSION TEST.

[Cameron]

First, make sure it isn't ignition related - swap plug and wire with known good, check cap.

Check what you are getting on a vacuum gauge (at idle, snap test, and holding steady RPM at about 3000 or so), compression test dry and wet, and finally leakdown.

If you do have to tear into the engine at least after these tests you will know what you are looking for.

[DUDONLY]

Check your brake booster......

HTH

[c.d.iesel]

and a VACUUM GAUGE reading at idle will tell alot

in addition to the compression & CylLeakTest

[yesterdaylight]

Thanks for the info guys - Have a new set of plugs in there so I dont think thats it. I have a vacuum gauge so I'll take some readings - what can i expect if things are awry?

I don't have a compression/leak down tester... I'll check into that...

[eurotrash]

Throw Pertronix failure in the mix of past #4 miss causes. Don't know if you run it. But I'm not the only one it's happened to. I'm running points now..

[jgerock]

Throw Pertronix failure in the mix of past #4 miss causes. Don't know if you run it. But I'm not the only one it's happened to. I'm running points now..

+1 on this inspection

PLUS remove and closely inspect your coil and plug wires. BTDT

[TobyB]

hey look, a bosch plug wire.

at 7500 with stock springs, I'd do a leakdown test to see how

badly bent the intake valve is.

If you have stiffer- than- stock springs, maybe not.

Still, do the leakdown test.

t

[Mike Self]

1. Just because the plug is new doesn't mean it's functioning properly...try another plug (and wire too, but not at the same time--always make one change at a time so you'll know what the problem was when you cure it).

2. Carefully examine the vacuum nipple on #4 intake runner. If you've removed all the emission stuff, it's probably been plugged. If the plug is loose or has fallen out, you'll have a massive vacuum leak to #4 cylinder and resultant rough running due to a very lean mixture. Then the head gasket will blow because of the overheating brought on by the lean mixture. Ask me how I know this!

Hope it's something simple.

mike

PS--7500 rpm isn't smart with a stock head, cam and valve springs. That's over 1k above red line. M10s are tough, but you can mess 'em up.

[eurotrash]

Yeah.. I missed the 7500 rpm part. OUCH.

watch this video.. its enlightening. Hurry, before it's pulled down..

[c.d.iesel]

healthy vacuum reading at smooth,

properly adjusted idle mixture, and

about 750 - 850 rpm should show 23 ...24 inch

with a steady needle

read more here -=> http://findarticles.com/p/articles/mi_qa3828/is_200108/ai_n8978956/pg_2/?tag=content;col1

[Cameron]

healthy vacuum reading at smooth,

properly adjusted idle mixture, and

about 750 - 850 rpm should show 23 ...24 inch

with a steady needle

read more here -=> http://findarticles.com/p/articles/mi_qa3828/is_200108/ai_n8978956/pg_2/?tag=content;col1

In addition to this, when you snap the throttle open, it should drop to zero, and then rise above your first reading, then return fairly quickly.

If you are getting a flucuating needle that bounces around 3-5inHG from your initial reading of 23inHG, and then smooths out as the RPMs increase slowly, you are looking at poorly seating valves, worn guides, etc.

A steady needle that fluctuates more as RPM is increase would lead to springs.

I would bet you are looking at valve seating issues if low vacuum from a leak is not the culprit. At work, when I'm dealing with a misfire, I tend to hook up the vacuum gauge first, because it takes so little time to test, and can tell you so much if you know what you are looking for.

[yesterdaylight]

Thanks for the great info guys. Borrowed a compression tester from a friend so I'll check that today. We'll see if I bent a valve.

I suspect an early downshift to second to save me from my fading brakes may have done me in but we'll see tonight.

Ian