Ozz1967

iTrader: (4)

84 T/A, CC Qjet, 700R4.

The car ran fine most of the year until the past month or so when temps started soaring into the upper 90's and low 100's here. It seems my RPM's have jumped 100 or so and the car now diesels just about every time I shut it off. Sometimes it just a stutter, some times it's a cough, but about half the time it's serious run-on. Obviously, if I leave it in gear and shut it off, it doesn't do any dieselling, but I'd prefer not to do that all the time.

Other than this, the car seems to run fine. Any ideas on what's causing it?

RedLeader289

iTrader: (2)

Have you tried lowering the idle speed a little bit and seeing if that fixes it? Since you said it's running about 100 Rpms higher than usual.

Mine will diesel if I don't have a high enough octane fuel in the tank. Have you changed to a lower grade fuel or anything?

OrangeBird

iTrader: (1)

This is the result of something in one or more cylinders getting hot enough to continue to ignite fuel even after the spark has been removed . The things that can cause this are carbon buildup in the combustion chambers or even an excessively lean mixture overheating the spark plug's electrodes causing them to glow . Is it running so rich as to build up carbon or so lean as to overheat the plugs ? I also wonder if your carb is functioning 100% because I thought a computer controlled Qjet was supposed to cut off the idle fuel supply with a solenoid at key off specifically as an anti dieseling measure ?

mjdavis68

iTrader: (1)

i thought it was related to timing being off and the result of the extreme heat from that makes it do that? idk jmo

Ozz1967

iTrader: (4)

I run 87 octane, the mid grade. I haven't touched the idle set screws or fast idle or any of that. As previously mentioned, it ran fine until it started topping 90* outside daily.

ASE doc

I would say that posts 2, 3 and 4 all make valid points. The chief culprit IMHO is overheating of the combustion chambers which can be caused by timing too advanced or fuel mixture too lean, to a point. The problem can be aggravated by too high idle.

The carbon build up isn't necessarily from too much fuel. It is simply incomplete combustion and some of it is normal. Too rich AFR would cause a cooling of combustion temps. Excessive build up can be caused by numerous things. Oil getting past the rings, not enough ignition KV, alot of short tripping, not letting the chambers heat up enough on a regular basis. No carbon deposits at all would be a bad thing as it would mean you're running lean enough to damage parts.

A couple of simple things to look at for a solution would be the heat range of your plugs. What works in the cold of winter may be too hot for the heat of the summer. Maybe drop back one step for a compromise. Look at timing. You may need to pull back a little, at least on vacuum advance(if you're using it) it's more the vac advance that affects lead at cruise. If you're running stock HEI, you may just have to pull the base timing back 2 degrees or so.

Is the EGR working? It's function is to cool down combustion. If it is deleted, this problem can result, especially in the hottest weather. Cooler plugs and a point or two of octane may help some there.

Mid grade, at least in my area is 89 octane. The cheap stuff is 87. Think about stepping up to 89 or 91, at least in the summer. Of course, higher octane burns slower and cooler, especially with today's, ethanol laden(high octane, low hectane), crap fuel. If you're having issues with carbon build up, more octane may make things worse, but it's something easy to try.

Can you control fan on temp? Or better yet, consider dropping thermostat temp 10 degrees or so. (194-180, 180-170 or whatever). Look at the plugs, are they white and/or scrubbed clean? If so, turn the fuel up a touch if you have the ability.

Spaceboy1980

iTrader: (6)

I had a car that I had to up my octane in the summer to prevent run on or dieseling.

ASE doc

It doesn't help that in some parts of the country, they increase the ethanol content(oxygenated fuel)in the summer when more fuel is burned, which causes the engine to run more lean.

I feel like I need to correct some of what I said before about the properties of gasoline fuel and ethanol. I mentioned the ratio of Octane rating to Hectane, which is actually Heptane. That is in fact not really accurate either. I wish I better understood the chemistry. My son the university engineering student rattles this stuff off like it's common knowledge. The more correct way to differentiate ethanol fuel from not ethanol would be to compare the calories(heat value) vs. octane rating. Hydrocarbon has higher calorie value than ethanol. When ethanol is used to raise octane rating, calorie value decreases, which is like reducing fuel input. Leading to a leaner fuel mixture. This is not to say that we can't make power with ethanol fuel. It just requires more fuel. Hope that all makes sense.

RedLeader289

iTrader: (2)

That makes sense, I've heard that before (being a former University Engineering Student myself lol).

ASE doc

Smart *** geniuses anyway. I'm sure I could understand the chemistry better if I could sit down and study it for a while without getting blurry eyed.

RedLeader289

iTrader: (2)

I remember saying similar things like that...after numerous attempts I've decided chemistry is just not going to click with me though, haha (which is why I'm a civil engineer haha). More power to ya if you do

naf

iTrader: (7)

I suspect the key here is the increase in idle rpm. The run-on is likely a symptom.

Check that the throttle is resting on the curb idle set screw when it's idling high. Ensure that neither the idle bump solenoid nor the fast idle cam are keeping the throttle open.

Next check for vacuum leaks, first in systems that open a TVS as the engine warms, like Thermac or EVAP.

Ozz1967

iTrader: (4)

Changing from 87 to 91 octane seems to have helped. I haven't really had time to dig into it any more than that at this point.

Thermac has been disconnected and plugged for years since I put my headers on.

MintWS6FORMULA

Engine deposits can cause this, because they soak up gas and all those fumes will be in the combustion chamber providing the fuel to keep running. Run a high quality fuel system cleaner through it.

aramirez121

use sea foam dump it into the intake theres a post on here i cant find atm but it will clean it out i did it to mine and it has a huge power improvement