StoneG

Hey guys and gals, new here, My names Stone.

My girlfriend has a 98 Grand Cherokee Laredo, I believe it has the 4.0 I-6. I want to do a good ignition tune up. Doubt it's ever had one, on cold mornings it takes a good couple of seconds to start.

So I figure a good tune up(hotter coil, new plugs, bigger gap, and possibly a timing bump) is in need. I've done this to my 91 Ford Bronco and it woke the beast up real quick. It's called the "Six Litre Tune Up"

Anyways I'd like to do that but a more mild tune up since it's just a daily driver rig with a lot of miles.

My main question here is what are the best stock plugs, I don't want some "fancy" platinum labeled plugs. Just some that get the job done correctly and last long. Like for the six Litre tune up we just use "Motorcraft Copper Core" cheap and solid. lol and what is the gap size for a stock engine?

Also the ignition coil and wires, I'm sure some cheap wires will do, but I want to know what you guys are using and getting best performance out of.

Thanks for any info and thanks again for letting me join the forum!

firehawk618

Welcome to the forum.

I am by no means a 4.0 expert but IMO you're best off just using stock / oem parts.

I do not believe you'll gain anything buy making the ignition "hotter".

Also you cannot advance the timing on these engines.

FYI you posted in the XJ cherokee forum, not the Grand Cherokee forum but since the engines are virtually the same I don't think that matters much.

tjwalker

Champion copper plugs are your best bet. They are inexpensive and work well in the stock 4.0 engine

I'd spring for the premium plug wires, distributor cap and rotor (brass contacts on the cap and rotor)

I'd recommend doing NOTHING with your coil. OEM coils often last the life of the vehicle. I've had (and seen) nothing but trouble with the Accel and other fancier coils; they have poor reliability and don't buy you a thing. But don't take my word for it; search will tell you the same.

Always a good time to review "fluids" when considering a tuneup. Coolant (and hoses and stat if it's been a while) transmission fluid, and fluids for the t-case and differentials.

Good luck!

salad

In addition to the above, consider that your OBD-II Jeep uses a lot more sensors than your Bronco did. Some of them are regular maintenance items, like the O2 sensors. Others should just be tested to verify they are accurate, like the intake air and engine coolant temperature sensors.

A handy way to blast carbon out (admittedly handier in the summer than winter) is using water to steam-clean the combustion chamber. There are a bunch of videos on YouTube of how to do this. I'm a fan of the spray bottle method.

Taking time to start may be caused by tuneup items, but in most of our Jeeps there's also a check valve in the fuel pump assembly that bleeds off fuel back into the tank when faulty. Try turning the key to RUN a few times before starting.

Bustedback

Opening the plug gap will do more harm than good. The larger gap will cause the ignition system to run at a higher KV load and wear the whole secondary ignition system out sooner. Today's fuel actually requires a smaller plug gap because there are less hydrocarbons in the fuel to help carry the spark across the gap.
My last tune up I used a tune up kit from Accell, got it from Summit Racing for about 70 bucks. It includes the plugs, cap, rotor, and wires.

StoneG

Fire hawk, I didn't know you couldn't advance the timing at all. I just figured that a good tune on my truck would be the same as a good tune on her jeep. Obviously not lol thanks for the response. And sorry about posting in the wrong spot lol

TjWalker, thanks for the info, just what I wanted to hear, copper plugs and brass contact. I probably won't do anything with the coil unless it still has the same starting problem.

And she's been pretty good about her fluids, but I'm sure it's been awhile. Anything else I should look at that would be good to check while I'm doing all this?

Salad, I guess that answers my last question lol I'll be sure to go over a lot of stuff. The jeep runs strong so I'm not too worried about random sensors, but I'm good with a voltmeter so I might as well go over most of them. Also you'd be surprised how many sensors my truck has lol but I'm sure this Jeep's a lil more complex.

About the steam cleaning idea, I don't really trust doing that. I know only an idiot can mess their car up doing that, but Idk about all that. lol then again I've never personally tried it.

BustedBack, thanks for the good info, lol on my truck it's a little different. I should've figured this car would be a different. I saw that tune up kit last night on summit. But didn't wanna buy it til I talked to you guys about all this.

dave1123

One other thing. Check all the vacuum hoses for hardness and cracks. Any minor vacuum leaks will cause hard starting in the cold. The main ones I would check are the crankcase vent valve and the MAP sensor.

Also, there is an unwritten rule in auto mechanics; Motorcraft in Fords, AC in GM, and Champs in Mopars. NGKs seem to work in most, but the engineers who built and tested the motors used their own brands and they seem to work the best.

Case in point; I used Motorcraft plugs and wires in my wife's Dodge K-car and they lasted a week before I started having problems. I went back to Champion plugs and a good set of Denso wires and the problems stopped. JMHO.