Alright fellas, need some help. It's with a 2000 Ford Explorer, belongs to a friend at work. He needed front brakes (pads/rotors/bearings), had an exhaust leak, and said the idle was surging so bad at idle it would stall out at a stop when in gear, had to put it in neutral to keep it running. Things are kinda tight for him, so I offered to help.

So I did the brakes, no problem. Opened the hood, found the EGR supply tube (this is a 4.0 V6 SOHC BTW) hanging, both ends rotted off. Figured that would lead to a surging idle, sucking in fresh air into the intake plenum at idle. Before fixing that, it showed four codes. P0402 (egr), 0171 + 0174 (both banks lean), and p0305, cyl 5 misfire. Cleared all, fixed tube, still had surging idle, hard starting when slightly warm, and had p0300 code after driving 30min, mixed driving, random/multiple cylinder misfire. Pressurized intake, found the seal for EGR intake tube was leaking, fixed that. Also pulled plenum, cleaned all oring seals, found some dirty grounds around the coil pack, fixed. Cleaned MAF sensor, EGR valve (seems to work), AIC motor. Put back together, went for another test drive. Runs excellent, except for the surging at idle is still there, only when stopped in gear. When I pull the MAF plug when surging, it seems to clear up for maybe 2 seconds then die out.

I haven't cleared the 0300 code, and it went away on it's own. Kind of at a stall right now. Any help is appreciated, I'd like to help the guy out instead of going to a dealer. I posted all information on another forum, figured I'd ask you guys to though. Any chance the AIC motor is no good? It idles fine (just under 1k rpm) in park/neutral, then takes a few seconds before it starts surging. Seems to cycle, surge gradually to almost stalling, then level out, then back, etc. Thanks in advance.

 

Tap the MAF when it is surging at idle, if it smooths out or dies then suspect a faulty MAF sensor.

 

sensor cutoff. if no response, possible crack with vacuum. check lines for dirt blotch on otherwise clean area. 2000 ford? spray the whole dang engine with WD40. it'll run like a charmed bull.

 

Wife's (girlfriend at the time) car was doing this. It would surge at idle and then sometimes quit when under a load when the A/C motor came on. She had it to the dealer a few times and they finally said they couldn't fix it. I was going to pull the valve cover to check the valves thinking maybe she had a tight valve or two when I noticed the plug wires were swapped on two and three. I swapped them back and the thing ran like a top. It had enough wasted spark to actually fire on the combustion stroke and run ok.

Your friends problem sounds like an idle control valve or intake leak to me though. A little concerned with misfire codes. Could be a crank trigger or a bad coil pack too. Good luck with it and post the fix once you solve it.

 

All plugs, wires (cap & rotor) are good? Fuel filter new? I know, sounds too simple but they contribute.

 

AIC was my first thought. Had a couple in the shop recently with that issue.

 

Thanks for the tips. Wires and plugs look like they were changed sometime in the past few years, no cracks and have hand written numbers on them. Good clean connections on the coil pack, which might be original, all connections (now, dirty connections for ground) are good and clean now. I did a quick lookover again this morning, PCV hose was a little sloppy. While taking off the plenum I found the vac. hose going to the fuel reg/shutdown (ties into a valve between fuel line and fuel rail, not sure what it exactly is) was barley on, that may have been misfire issue? It does run smoother now when stepping on it.

I'll keep looking though, keep the advice coming, just going on the fly here! Will check vac lines again, most seem ok and I got a good view with the plenum off. Just hard to think that it was running at all with the EGR tube OFF, leaking intake EGR tube, and the other vac issues I found. Thanks again.

 

If the coolant temp sensor for the ECM is faulty, it can make the computer think the engine is warmer than it is and lean out the mixture, causing surging... Bad 02 sensor can also cause surging but I would think would have tripped a code.

I would also suspect IAC...

Have you run a compression test?

 

No have not run cylinder compression test. I'm gonna go ahead and guess the early "lean" codes were from the EGR sucking fresh air instead of exhaust. Now I'm still coming up with P0300.

Just noticed, in neutral, idle kicks up when a/c kicks in. In drive, darn near stalls out. I tried tapping the MAF, nothing, unplugging it just kills it. I tried snugging down the intake plenum bolts (I hate plastic!) and when tightening one of them I thought I noticed it bog a little bit harder, could have been imagining things though. Definetley surges more when warm, after the idle kicks down. The P0300 also seems to go away when driving. Does anyone make a points and carb retrofit for this?

I double checked every vacuum line, no leaks. Will check that intake gasket again tomorrow. Probably will get new coil pack, wires and plugs/fuel filter, due for a tune up anyhow. He hasn't done anything (besides oil changes) in almost 5 years. Then AIC motor, then I don't know. Thanks again.

 

It would also be interesting to see what the intake vacuum is doing when it is surging.

Perhaps there is still an intake leak somewhere. Power brake booster, PCV valve, gasket somewhere, or something...

 

With the engine idling try spraying some WD40 around the intake plenum and take note if the idle speed varies while doing this. You could also do this around any vacuum area or parts. If the idle speed changes while doing this, that is your clue.

Have you tried bumping up the idle speed by about 100 rpm and seeing if the problem goes away?

 

I agree, I would clamp off the vacuum line going to the brake booster and see if that does anything.

Usually a p0171-174 on a 4.0 sohc is due to intake gaskets, both upper (plenum) and lower. There is a tsb repair package available for this. It is also an extremely common problem on these engines.

 

OK just got home...

Checked vacuum from the power booster line, when happy idling in neutral it's around 15-16in vac. Once dropped into gear (this is warmed up now) drops down to 14~ in. If the A/C kicks on, it can get down to around 8-9 in for short period of time. With a/c off in gear, "normal" light surging from 13-14 in. The surging gets worse after a few minutes, and will spike down to around 10.5-11, and bounce between there and 14 until it smoothens out at ~14 in. High idle was from 16-18in, never bounced higher than 22ish. Idle in neutral is just under 1k, in drive it starts running rough around 500, seems to like 700ish. Gonna let it cool down, pressurize plenum again and really check out around the plenum seal again with some soapy water. MIL light came on this time with the P0300, was off with the code yesterday.

FWIW, I talked to the owner again today, the motor has been changed (junkyard ), which may explain why the EGR tube for a OHV motor worked instead of SOHC. Radiator brace label states SOHC, valve covers look awefully small and recessed for it to be anything but a pushrod motor to me.

 

It really sounds like a vac leak, i agree with the previous posts, spray some carb cleaner around the intake and some of the major vac spots, i have found countless problems with this, i didn't see where you checked the tps.

Dar

 

My wife's 2002 Mustang with the 3.8 V6 had a bearing go out on the camshaft position sensor (basically the replacement of the distributor when the coil pack came around). I replaced the whole shaft with the sensor as well. About a year later it started surging at idle somewhat like you mentioned. It actually stalled on her one or twice. What I mainly noticed is the RPMs would drop about 200 - 300 RPM when the A/C compressor clutch engaged or when coming to a stop at a red light. I cleaned the throttle body and AIC with no improvement. It did this for a month or so and eventually threw a code (camshaft position sensor fault). I still had the original sensor (sensor and shaft are separate). I slapped it on. Not only did it clear the code, but solve the irratic idle as well. Part of my point in explaining this in detail is that it ran similar to how you are describing for quite some time before throwing a code. It would be worth looking into...