tii spark issue

[hiebertm]

I need help as I'm zeroing in on my tii's misfiring/bogging problem:

When I start the tii I get good spark on 1,3,4,2 (observed w/ the inductive pickup of my timing light). When the revs start to drop off because the car is warmed up, then cylinder 1 and 4 start to miss. The miss is worse at lower RPM. After a few more minutes, 1 and 4 missing gets way worse and then 1 and 4 completely stop firing so my idle drops to a couple hundred RPM (if I rev back to 1500 RPM, they fire but miss often). In all cases, 2 and 3 fire fine.

Things I have tried:

- swapped the distributor cap for another used cap I found.

- moved the plug wires around and the problem stayed w/ cylinder 1,4.

- put a new plug in 1, no affect.

- checked timing at ~2400RPM, and I'm close but not perfect because the revs are bouncy due to the miss.

- I see 11V at the coil, doesn't seem to change between cases when I have spark on all 4 or when I have spark on just 2.

- I've checked for play in the distributor shaft... but I'm not sure what's acceptable. There appears to be small vertical play ( <1/8 inch) but no horizontal play.

I have the tii mech advance dizzy and petronix.

I'm a newbie mechanic, so I'm comfortable w/ the fact that I know nothing...any education or advice is welcome! What would cause persistent missing on 1 and 4 but cause no problems on 2,3?? Can a bad coil do that ( I would think that the coil couldn't cause cylinder-specific problems...)? Is this a petronix symptom that anyone has heard of or does this indicate that I need a new distributor?

Mark

[Mike Self]

1. compression on the misfiring cylinders when the engine is warmed up? A cylinder that has marginal compression cold may lose enough warm so it's just not contributing to the engine's operation.

2. to see if #1 & 4 plugs fire if you remove 'em from the cylinder, hook 'em up to their plug lead (grounding the plug on the cylinder head) and having someone hit the starter while you observe the plug sparking. You should get a nice fat blue spark. A wimpy yellow one = ignition problems somewhere.

3. If #2 proves to be true, have you tried a nice new dizzy cap and rotor, and then go back to points (remove the Pertronix temporarily).

That'll at least get you started on the road to solving the problem...

You need to check one thing at a time, in logical order, to track down the problem.

mike

[PatAllen]

the miss may be due to a small leak in the air tract, causing a lean mixture ?

actualy on any car, when you can alter the mixture at idle (think at a flap door air meter) there is a point where only one or two cylinders start to miss, meaning that (EFI world) there is a bad injector on that cylinder, a leak at that cylinder intake, a flow problem (carbon deposit) or last but not least, a weak spark...

just an idea...

what kind of wires do you have ? with the oem "solid coper" wires and the petronix, id suggest you to find modern age wires with carbon inards.

Ive seen several cars with points/petronix upgrade start to throw flames out of the wires because now the spark is too intense for the kind of wires they used to have.

just a share...

[hiebertm]

Pat,

I have fairly fresh bosch 7mm wires, I think. I'll think about adding better wires to the list since I'll be refreshing ignition components soon apparently.

So I need to ask a dumb question:

I was thinking of the ignition system as independent... that it throws a spark through that plug regardless of pressure or fuel mixture in that cylinder. For my old-tech 2002, am I wrong to think of the spark delivery as independent of conditions in the cylinder (as long as the plugs aren't fouled)?

As Mike suggested, I'll check the spark outside of the cylinder tonight to be completely sure that the spark is bad on 1,4. I'm pretty sure that when I'm missing, the spark is missing.

[Richard]

The missing starts with the warm up regulator coming on or not....overly rich will not run either and mimics bad spark......

[jgerock]

When you installed the Pertronix, did you leave the ballast resistor in place located on top of the coil? Since the Pertronix wires are really thin, did you check to see if a wire is broken.

How old are the plug wires? Some of the Bosch copies don't have good screw-on end terminals/boots.

Since your car is a 72tii, does it have the black plastic intake runners. If so, these leak like there's no tomorrow.

Other leak points that would cause the tii to misbehave: Valve cover gasket, vent line to air cleaner with tee to throttle body, Kugelfischer make-up air hose connected to bottom of intake plenum.

[hiebertm]

So checked the spark against the engine block. I verified that I have zero spark on 1,4. Spark maybe looks a little weak on 2,3 in the same test. So the miss is definitely an ignition issue, not mixture/vacuum leak.

Also, I found a new rotor and cap w/ the PO's box of parts, and swapping these didn't change anything.

Jim,

Plug wires were done by the PO; and the problem stayed on the same cylinders when I moved wires around so it seems like they aren't the problem. Ballast resistor is still in the circuit. No obvious broken wires or loose connectors, but I need to take a closer look.

So I'm thinking next steps:

1) Carefully inspect the petronix wiring and make sure nothing's loose

2) Go back to points temporarily to see if it resolves the problem

3) Try swapping the coil

[ray_]

What plugs are you running?

Don't run platinums in a tii.

GL,

Ray

[jgerock]

I agree with checking the spark plugs (type and condition). Only use Bosch W7DC if you can find them (resistor type OK - that's what my car currently has). Tii's are sensitive to spark plugs. The late, great Dan Erwin wrote an excellent how-to article (Bimmer # 35 June 2003) that mentioned (among other things) his 74tii wouldn't run until he changed the plugs ... "they looked new".

Please check for strange vacuum leaks.

What about your brake booster check valve...?

[hiebertm]

Plugs:

I'm using the NGK equivalent of W7DC, and I've also tried 1 code hotter. Always non-resistor type. Boschs are hard to find around here but I should get around to ordering some by mail.

Checking for vacuum leaks:

the plastic intake runners are no longer on the car, so I'm not worried about them anymore. I do think that I hear a hiss using the "listening through a hose" method, near the valve cover vent line and possibly other places.

But regardless of whether I have vacuum leaks, I have zero spark on 1,4, tested against the engine case, after running a few minutes. So the first problem to solve is in my ignition system and can't be related to mixture/vacuum/compression... right?