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-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?
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
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.
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.
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.
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).
). I’ll check the resistances in the morning, but I’m guessing a new IAC may be in my future?