A Stumper. High RPM, Top Gear Engine Miss

Hello!

I just got my 225 V6 Jeep back to a drivable state after many upgrades. Headers, DUI HEI, 33 inch tires and an overdrive. Also a new radiator.

The short story, the Jeep has a high RPM "pop" or miss, but only in top gear (3 speed) or in 3 with the OD on. At low speeds and the other gears, it runs without a miss.

This morning, I've tried:

Changing the timing (both power tuning and with a light)
"Hot wire" the distributor (bypass the ballast)
Checked resistance of plug wires

Checked plugs on the passenger side bank. The front looked a little sooty, and the back two were consistent with each other.

After I warm up and eat something, I'll go back out and do a compression test...

My gut says its valves or a sticky lifter or something, but it runs just fine at low RPM! It even lugs ok. It ran just fine in top gear before when I parked it 6 mo ago. I have 1/2 tank of fresh gas in it.

I searched last night and heard mention of adjustable vacuum advance tuning or changing springs. This also doesn't fit with the one spark plug being a little sooty...

Anyone have any ideas?

 

Checked with ether, unplugged the PCV valve and put in place a bolt to check for leaks...

Also got a new piece of vacuum advance tubing.

Soooo I put in the old distributor (points!). Ran fine, no pops.

Something in or about the DUI is funny. Could be the electronics died and it went into a "emergency less than optimal run" mode? Could it be that its tuned for an even-fire V6? I didn't think it would run....

It's crazy!

 

You said you had a ballast resistor in line with the distributor? Not real familiar with the DUI unit but all HEI's I've worked with don't use a ballast resistor. Could be the voltage is being stepped down too far for the load creating spark issues. I'd check for the other things the fella's have said already... Nickmil

 

Reads to me like he has already tried running the dizzy without the resistor. Would running at an undervoltage cause evil electrons to attack the ign module in the HEI?

 

Sorry for the delay--

No balast wiring when I was running the DUI.. I ran a direct wire (like 10 gauge) from the + side of the battery to the + side of the distributor. I wanted to bypass all Jeep wiring, ignitions resistors and all.

Putting in the old points based distributor, and all the balast wiring back in place made the Jeep run just fine for about 250 miles from my parents shop to my apartment up here. Never skipped a beat. I left the D*** DUI in the box. When the distributor stops being in the time-out corner, I'll check for a broken/sticky/janky mechanical advance spring.. That was one place I didn't think to look!

After reading merls_garage link last saturday, I confirmed it was an oddfire cap. That site is a great resource for understanding why an oddfire is.. odd. Triple checking... the outside of the DUI box says "Buick Oddfire v6 225".... but then again it also says "easy quick bolt in!" R)

The vacuum advance on the DUI does not seem to have an adjustment allen so I can't even tweak the curve.

I've never seen an engine that will run just fine lugging around the farm, plenty of torque at low-mid RPM (in gears 1 and 2), and only sometimes have issues in only top gear and top gear overdrive.. It really only puked on freeway type situations, otherwise relatively undetectable... One more think to check if I buy a used jeep!

Hilarious tangent: when the old distributor finally went back in and everything appeared well on the test run, the jeep ran out of gas on the way home! Spare Jerry cans to the rescue!

Thanks everyone for the helpful insights. It's been a long 7 months working weekends at a garage far away from where I live.

 

Yep. Straight + side of battery to + distributor with 10 gauge, still ran funny.

I thought maybe the gauge of wire might cause current problems, since it looked kinda small... Its entirely possible that its voltage sensitive, but usually with voltage dependant electronics (from my experience), they do nothing or they heat up and die spectacularly when powered incorrectly. Automotive stuff is supposed to be extreme temp and relatively ruggedized. If it has linear regulators in it, I would think it would just stop functioning when undervolted...

I have essentially 3 or more hypothesizes:

1.) Its entirely possible the electronics puked (my jeep's initial wiring, infant mortality of the silicon chips, cosmic rays)... Considering the microcontrollers inside the silicon, its possible tuning parameters DUI has done to the curve has reset back to a untuned state. Usually when these things fail, they just shut down and the engine doesn't run at all though..

... in which case I do not want it anywhere near my Jeep.

2.) The cap is ok, but the internals (weights/silicon/ mysterious green elves escaped) are actually for a slightly different engine. Something bothers me about DUI's FAQ says that any tower can be a #1, but I don't believe this to be true due to the odd fire nature of the engine... (the 45/75 business, merl explains it excellently...) I found this after some pondering since the cap has no #1 marking on it.

3.) Something is janky with the mechanical springs. I wouldn't think this would be the case since it runs well in 1st gear in a similar RPM band. I did not check it however...

I'll post back if I figure more of this out.... R)

 

Another Jeep forum I visit has a gentleman who STRONGLY believes that HEI conversions on Jeep engines is asking for disaster. His knowledge certainly seemed to exceed mine on the subject, appears to be a credible guy. IIRC he had 3 or 4 major areas where he felt HEI was deficient, one of them able to destroy your engine. Having a DUI myself, I became kinda concerned...I went to DUI's website, and low and behold, they were aware of and addressed every issue he raised.

So I posted up, asking the guy, what he thought about the DUI brand dizzy, in specific. The gist of what I got from his reply post was that the DUI was one of the higher quality units out there, and he had no bone to pick other than the high price. As the guy is the biggest critic I have ever run across concerning HEI, and had no real complaint about DUI, I rest a little easier.

My point here is that DUI seems to be a quality product, a definite step up from an ebay special. It's possible you just got a bad one. I would give DUI a call. You might be surprised.

Interested to see how this works out, keep us posted...

 

I think I know the guy you are talking about and he is, well, opinionated on the subject. I find HEI to work very well on almost all situations with stock type motors. I'd certainly take a "tuned" distributor over a stock unit, but for the average user, a stock unit is just fine. I have the ability to fine tune a distributor and I've been using the stock HEI unit in my 231 since I installed the motor in 1989. NO issues. I've raced, rolled, rocked, sand dragged, and done about everything else to that Jeep and have had no ignition related issues other than normal parts wear. If I were running a blown, turbocharged, high compression, or otherwise super high performance motor then I would say many of his arguments are well founded. For a stock vehicle though the arguments are mostly unfounded. Nickmil

 

I would agree. From all the caterwallin you would think engines were being destroyed daily, the world over. In all of my wanderings in jeepdom on the net, I have seen one (1) post claiming engine damage from HEI. It was a post on the same thread. Not much evidence of a widespread horrific problem, IMHO.

That aside, I still wanna see what Clark comes up with from DUI.

 

So! I have been meaning to follow up on this story and give a post mortem..

Turns out, one of the plugs.... though visibly ok, had a cracked ceramic part on the insulator, causing the electrode to just barely be held in there. I knew it had to be plugs when it started high-RPM pop-ing when the old dist was back in...

I've never seen the center electrode just fall out of a plug like that... I didn't drop it or otherwise handle it roughly, but that doesn't mean someone else didn't! I can't recall if they were Autolites or NKGs..

I still have the old distributor with points in. I found it interesting how forgiving the points system was compared to the HEI DUI. I also found this problem very interesting because it would idle and run up to 40 MPH without any problems or noise whatsoever..

 

Wow, old post & thanks for the update. Never seen a bad plug problem like this before. Glad it was not a problem with HEI/DUI.

Thanks for posting.