Mx83 Misfire, Searching For Cause?

[Mr.Matt]

Okay,

 

1991 Cressida Grande.

 

Well taken care of, 6th cylinder has started misfiring (the one closest to the firewall).

 

I replaced all the spark plugs and ignition leads yesterday (and the air filter as I had to take part of the intake parts out anyway) to try to combat this issue.

 

And it worked fine, but today the problem has returned.

 

It's running a stock 7M-GE, which is burning off a small bit of oil and water, but no cause for alarm yet (I'm not needing to top it up every day and there's no blue or white smoke coming out of the exhaust).

 

There was a bit of oil on this specific spark plug when I took out the old one, and it was abnormally tighter than the others (the spark plugs had not been replaced in a while).

 

Any other info that is needed just ask me, but I am looking for any possible causes/solutions.

And can anyone tell me if this misfire is causing damage to the engine? I do still need to drive the car, so I have just reduced it to a minimum and not driven it hard at all.

 

Thanks for reading!

Edited March 27, 2012 by Mr.Matt

[madKE35coupe]

Have you checked the coilpack yet? Not sure whether it could cause that or not?

[ke70dave]

hmmm interesting.

 

when every i have a drama with 1 single cylinder playing up, i try to eliminate anything.

 

so spark plugs and leads have been taken care of.

 

swap the coil pack son 5 and 6 and see if 5 stuffs up?

 

next step is compression test, and see if clr 6 shows up anythign funny...

 

perhaps your oil or water is going into cylinder 6 (headgasket, dodgy valve stem seal), causing clr 6 to misfire....

 

only guesses really.

[altezzaclub]

If you replaced the leads I assume it has one coil & a distributor with leads like a 4K.

 

Is there any sign of the dizzy shorting out at lead #6? A crack or a black line running across the inside or the outside. Pull the lead off when its running and short it out against the motor to see the spark jump. (make a mistake and you will jump too! It should be OK with new leads... )

 

Do a compression test and check that #6 is OK, you might have a burnt valve.

 

I doubt the misfire will cause any damage, everything works as usual except that cyl doesn't fire.

[Mr.Matt]

Okay status update.

 

Took it back to the mechanic, who found that no. 1 was not receiving any electricity, so we swapped back to the original lead for no. 1.

 

Which seemed to fix it. The intention being that if the fix seems to last then go get the new lead for no. 1 replaced and be done with it.

 

Now the old lead is doing the same, considering it has now changed spots, and there is next to no oil at all in no. 1 at all (the plug came out dry as a bone),

 

I'm guessing now the distributor is on the way out?

 

Any thoughts?

Thanks for the replies so far.

[Mr.Matt]

Okay, it has been suggested that the distributor cannot be the cause.

 

Just a Recap.

 

#6 went and a symptom was that there was more oil then the others had, but that wasn't a definite cause for the misfire, just an assumed cause as no other evidence was present (and we didn't think to check if the cable was getting any electricity or not).

 

#1 then decided to go, but #6 was fine, when checked with the mechanics lead tester, we found that the ignition lead for #1 was not getting any electricity (at the time).

 

Once again not definite, but it was assumed the cable was at fault. Now that swapping cables fixed the issue for a little bit, but the problem with #1 did return. Going to the next component would logically lead to the distributor wouldn't it?

 

And how do I do a compression test?

Edited March 28, 2012 by Mr.Matt

[altezzaclub]

You could wait until the problem occurs, then spray the dizzy cap with CRC or WD40 which act as insulators. If that instantly fixes the problem then you know where it is.

 

For a compression test you buy a gauge, take out all the plugs, have someone sit inside with the throttle wide open and while they crank the starter over 5 or 6 times you hold the gauge in a plug hole. The compression will shoot up the first few strokes then stop going up at its maximum compression. All cylinders should be within 10psi of each other, and they will range from 160psi for a good motor down to 120psi for a worn motor.

 

here- every guy needs one! The push-in cheap one or the screw-in one with a hose.

Edited March 28, 2012 by altezzaclub

[Teddy]

If its done what most 7M's have by this point in time, the head and or the block has corrosion & coolant slowly making its way into the cylinder will be the cause of the missfire.

 

But, do the compression test (altezzaclub's post above, fantastic) first to rule out my theory. '

[altezzaclub]

Ah- Teddy may have hit it. If they have a tendency to corrode the water will make it hard to fire up.

 

The compression test quite likely won't pick that up, it probably leaks too slowly, what you need for that is a leakdown test.

 

You buy the one with the screw-in hose and attach a compressor to it so you can pressurise a cylinder to 100psi, then turn the compressor off and watch the pressure drop. If it leaks into the water jackets you will see bubbles in the radiator, or if it leaks into the sump you can hear it as you see the pressure dropping.

 

I had leaking exhaust valves and we could hear it leaking into the exhaust quite clearly.