Hello all, I’m having a bit of car trouble I could use some help with. I’ve got a 1998 Jeep Grand Cherokee, 6 cyl. engine, ~145k miles. A few weeks ago, I noticed the engine would occasionally run rough at idle, dropping down to a low rpm (~500 rpm, but erratic) for a few seconds before stalling. This usually happens when in park or stopped at a stoplight, but it’s occasionally happened at low speed while coasting to a stop (never while accelerating, or while traveling above 20 mph or so). Generally, the car will restart right away, though sometimes I have wait a few seconds to a minute (I was a real popular guy at that intersection).
At first, no codes came up, so I changed the plugs and wires (I was due anyway). That didn’t help. (The plugs, while worn, looked normal to me-no residue). I’ve since had a P0351 code pop up. Before I throw parts at this code (ignition coil, I believe), is there anything else I should be looking into?
Thanks in advance.
Replace the coil and clean the IAC and MAF.
I don’t see a reason to replace the coil if the engine runs fine otherwise.
And as far as a MAF sensor, the engine doesn’t have one so there’s not one to clean.
What I would do is replace the Idle Air Control valve.
The IAC valve controls the engine idle speed whenever the gas pedal is released.
Thanks-how difficult is the IAC valve to replace? Is it something that can be done with a modest set of tools and knowledge, or is it a fairly involved process?
Replacing an IAC is usually pretty easy. I’m just talking in general, not about any particular vehicle
I’m not that familiar with the Jeep 4.0 straight 6 . . . does the 1998 model have 1 single standalone coil and a distributor . . . ?
Or does it have a “coil pack” . . . ?
Here’s something interesting. Different model and year, but very similar engine
Thanks-that doesn’t look too difficult at all. @db4690 , the 4.0 has a single coil and a distributor-beats replacing 6 coils, if that were the issue. Thanks for the link too. @knfenimore, thanks for the tip about the ECM-hopefully it’s not that, but it looks like refurbs are <$200, so it could be worse.
It looks like the coil is about half the cost of an iac valve, which in turn is about a third of the cost of an ECM. Looks like I’ll march on down the line. I’ll post back with an update once I get this nailed down, or if none of the above work. Thanks, everyone, for your assistance.
Thanks for the information about your ignition setup
Now that I know you’ve got a distributor with an individual coil, I can see that my link doesn’t apply
Anyways, do you have a multimeter? You could at least see if the secondary and primary resistance is within spec
I’d be tempted to thoroughly clean the IAC and throttle body first. In fact, it might be a good idea to remove the throttle body and clean it on your bench, to do the best job possible. Couldn’t hurt to give the IAC passages a blast of throttle body cleaner, also
I do have a multimeter, and I’ll check into that. I’ll pull the IAC and give it a good cleaning too.
In case you don’t have it, from my Haynes Manual . . .
1998 and earlier distributor specs
primary 0.96 - 1.18 ohms
secondary 11300 - 15300 ohms
primary 0.96 - 1.2 ohms
secondary 11300 - 13300 ohms
I don’t even know if this is accurate, I have mixed feelings about Chilton and Haynes, but it’s all I’ve got for you right now
Thanks-those help a lot. I left my Fluke at work, so I’ll check resistances tomorrow. As an update, I’ve cleaned the IAC and replaced the ignition coil, with no effect.
I don’t have enough information to say for sure, but the problem seems to be getting worse as it gets warmer out. I’ll update with resistances when I have them.
For what it’s worth, I still consider the possibility the IAC is bad, even though you cleaned it
Just for kicks and giggles, what happens if you let it idle with the IAC unplugged?
Is power and ground to the IAC okay?
Well, I let it idle with the IAC unplugged. It idled at a higher RPM than typical (1250 vs. the usual 700-800), but it didn’t stall. (It also threw a P0505 code, which is not surprising in the slightest ). I’ll check the resistances in the morning, but I’m guessing a new IAC may be in my future?
EDIT: Actually, the car doesn’t stall when I’m giving it gas (keeping the RPM’s up). Since disconnecting the IAC made the engine idle high, is it possible the only reason I didn’t get a stall was the car was keeping the RPM’s up high with the IAC disconnected?
I know this isn’t exactly sound diagnostics, but since the engine idled higher with the IAC disconnected, I’ll assume power and ground to the IAC are good
A new IAC would be my next step, based on what I’ve read so far
Don’t get mad if I was wrong . . .
Can I assume the normal rpm at idle, with a warmed up engine is around 700rpm, plus, minus 50, according to the underhood sticker?
Like I said, I’m only marginally familiar with the Jeep 4.0 . . . an in-law has a 2004 Grand Cherokee 4.0, but it’s the next generation, and it’s got a completely different ignition setup. And I’ve only worked on it a few times
Hey, no worries-a new IAC’s only about $50-certainly not throwaway money, but it won’t break my back if it turns out not to be the solution.
The normal idle is around 700 rpm or so. (Your definition of ‘marginally familiar’ and mine are quite different!) I’ll still check resistances before grabbing a new IAC, and verify power and ground while I’m at it.
Thanks for all your help so far, and I’ll post back when I know more.
You’re welcome for any help . . . and good luck!
From what I’ve read so far . . . and the fact that you’ve got a Fluke multimeter, and probably retrieved that fault code with your own device . . . tells me you’re far from unprepared
By the way, you picked a good meter. I also have a Fluke at work, and the company really stands behind its product, lifetime, in fact. Over the years, it’s needed calibration a few times. I sent it in, they calibrated it, sent it back with calibration certificate, no charge
Thanks-I learned a long time ago that I’d spend twice the cost of a Fluke in cheap multimeters.
I left my Torx wrench at home, so after a bit of gymnastics to take the resistance with the IAC in place, I got a resistance of 54.4 ohms (!) for the primary (power and ground were good). I’ll grab a replacement this afternoon and swap it out.
I’m a little confused . . .
Those resistances I gave you were for the ignition coil, which you’ve already replaced, I believe
Sorry if I wasn’t very clear, earlier
Anyways, it’s good that you personally verified power and ground for the IAC
And I still see it as the next step . . .
Ah-I misread-sorry. I did replace the coil earlier, with no effect. (I was apparently a bit confused too!)
I just swapped out the IAC and let it idle 5-10 minutes. Steady as a rock! It’ll take me a week or so to be sure the problem’s gone away (and I’ll update here), but I’m cautiously optimistic. Thanks @db4690 , @Tester , and @knfenimore for all your help!
This almost falls under the category of a rhetorical question, but why did a faulty IAC throw a code for the ignition coil? There’s a perfectly good IAC fault code waiting to be used. (Granted, I know a given code doesn’t mean ‘Part X is bad’, but I’m curious how a problem with air throws a code for the electrical side).