4.0 Rough startup but smooth running?

I have a '96 4.0 with about 211k. Most everything is original, except the alternator, and the plugs/wires/cap were replaced about 15k ago.

When the engine is dead cold and I start it up, it sounds pretty rough for the first 5-10 seconds, and the idle stays about 1100 RPMs. I can feel the cab shake a little as it sounds like it wants to die.

After a few seconds, idle lowers to about 800, and it smooths out considerably. After that it seems to run great, good performance, and good gas mileage. If I restart it hot, it starts nice and smooth. If I restart it when the temp gauge is below 100, but it's not completely cold, it still starts smooth. Wait overnight and it's rough again.

Where should I start looking?

in the effort of not making another thread and tickin' people off I'm having the same issue and would greatly wish for some information as well. I also recently replaced my plugs and I just got done w/ idle issues w/ an exhaust manifold replacement - fixed it all up to idling good and now the jeep is doing this same kind of starting on me. I'm tryin to fix it before a trip that I'm takin this upcoming weekend any new help is highly appreciated.

To original poster: so I'm not a complete loser in piggy-backing, I've done some searching, but none of these threads seem to actually have a solution (or one that worked at least for me, maybe it might work for you):

http://www.naxja.org/forum/showthread.php?t=906529&highlight=rough+start

http://www.naxja.org/forum/showthread.php?t=899585&highlight=rough+start


http://www.naxja.org/forum/showthread.php?t=898771&highlight=rough+start

http://www.naxja.org/forum/showthread.php?t=65675&highlight=bad+start+rough+idle

http://www.naxja.org/forum/showthread.php?t=99888&highlight=bad+start+rough+idle


So please, if anyone can help, We'd both greatly appreciate it!

Leaking fuel injector? Not enough to leak much in operation, but while it sits and cools it dribbles and floods while cold but evaps if hot. Do you get any smoke on a cold start like it's running rich? Blue or gray smoke?

-Skip

again: I'm not trying to take over this thread, but I found this information helpful from this thread:

http://www.naxja.org/forum/showthread.php?t=908695&highlight=trouble+cold+start

first thing I'm going to do tomorrow morning is check my t-stat (altho it's been replaced) and see if it could be the culprit....

simple, the oil settles, and when started with settled oil, it takes a bit for the oil to get into the top end of the engine(pistons, rods, crank,ect) therefore it runs rough when cold

redyj said:

simple, the oil settles, and when started with settled oil, it takes a bit for the oil to get into the top end of the engine(pistons, rods, crank,ect) therefore it runs rough when cold

Click to expand...

I find that hard to believe. While yes, the oil does settle over night, it doesn't settle THAT much....there is still a fair ammount that is still on all of the internal parts. If it were to sit for a few days, then yes, all the oil would drain to the bottom of the pan. And when you crank it up after it sat for a few days, it shouldn't run rough. The only unusual noises you should hear would be the top end being temp. starved for oil (such as a knocking or tapping)....ask me how I know.

Getting back on topic, I'll be keeping an eye on this thread as I was having the same problem with mine, right before I dropped it off at the body shop last monday. Two weeks ago it acted up every day but the last two days I had it. I've hooked my scanner up to verify coolant temp being sent to the computer was correct, which it was. I'm hoping to have it back either late this week, or at the latest the very beginning of next week. It will be interesting to see how it runs after sitting for a couple of weeks.

I tested my ECT before leaving for work and the readings came up to be 14000 cold. Compared to the numbers from the other thread, it should be 7000 cold. I'll test it when I get back home to see what the hot readings are.

I did do something that helped though: I put the key in the 'on' position and just left it there for 30+ seconds. Then when I went to start, it cranked up all nice and normal. I'll be attempting that as well next time and if it works I'll start doing that until a fix can be found... still thinking about getting a new ECT though.....

the ECT is supposed to be 7K ohms or greater when cold, and 1K when engine has reached op. temp. Mine is fine... so the prob is somethin else.

I'm out of solutions so any help is appreciated.

Bump for OP....

you don't have a vacuum leak do you??

with that many miles, i would venture to say your CPS is nearing the end of its life (if it's original)

Vacuum leak would raise the idle. CPS would keep it from starting and/or not change after a few minutes.

It's sounding just like the description of a cracked piston given in another thread.

Also look at your spark plug gap. The cold demands more of the spark to ignite things, adn if marginal, may not be doing it right at first when cold. The cylinder is the first to heat up and last to cool down... Maybe a bad plug too.

Insert a wire behind each plug. Have someone start it cold. Quickly ground each plug one at a time. The one that doesn't change things is the problem. Maybe the one with the crack, or the bad plug, bad gap, whatever. Go from there.

-Skip