HELP! Rough idle/high rpms when stopping

Scott · #1 02-05-2010, 10:28 AM

Ok here's my situation: 97 2wd 2.3 Lima 5 speed, 150,000 miles. Blew a head gasket about 6 months ago. Tore her down, find out the head is cracked between the valves in cylinders 2 and 3. Buy new head and install as per specs. Also installed at this time: new plugs, wires, coils, water pump, thermostat, fuel filter, all short block gaskets,timing belt and serpentine belt. First turn of the key she fires up hallelujah. She runs good for about 4 months, then I start to run into rough idle. Started very faint but has gotten to the point that I have to brake and gas while stopped. Also the motor will rev pretty high (don't have a tach, but feels like 4-5k) when I let in the clutch to coast to a stop, so I have to keep it in gear to engine brake. After checking my handy dandy Haynes, decided I had an intake problem, and so far have replace the IAC and TPS, disassembled and cleaned the throttle body, and the same for the MAF. She's still having idle issues and high rpm coming out of gear, although after cleaning all this crap the throttle response is great.

Before you ask, I'm positive the timing is correct, double triple checked. One thing I'm not sure of is the exhaust. I'm running the OG O2 sensors, and my exhaust has a rattle at about 2 grand in every gear (I think its coming from the cat). I have a new upstream O2 sensor, I just can't unplug the old one to swap it in (any tips?). I can get to the connection, but it's in a very awkward spot and I haven't been able to disconnect. I did remove the upstream sensor from the exhaust pipe thinking it may allow the exhaust to escape better if the cat was clogged but the idle issue was still there.

The CEL shows insufficient egr pressure. I can't find any vac leaks, so I think the egr tube may be fouled up at the valve/gate where it comes off of the exhaust manifold. I don't think this would cause my idle issue though. One other thing to note, the idle issue usually doesn't appear immediately at start-up, she will run fine for a minute or two and then swan dive.

When in gear and under a load, the engine runs great, better than it has in years actually. Would a faulty O2 sensor cause this? Or does it sound more like a clogged cat? If the cat was clogged I'd think that unplugging the upstream O2 sensor would help diagnose that, but it runs the same.

I'm about to buy a new cat, probably the rest of the exhaust while I'm at it, but I just need to talk to someone about this. It's a lonely road trying to reverse engineer my beloved ranger, although I will say this and other forums have been absolutely extremely helpful. This thing is giving me gray hair, and I'm only 27. Any ideas O wise ones? Help a brother out!

Scott · #2 02-05-2010, 10:53 AM

Oh yeah I also installed a new pcv valve during head exchange.

RangerLuc · #3 02-05-2010, 10:55 AM

my truck wasnt that bad but it did have a rough idle and high rpm's like you said. i ran seafoam through her and now she idles like its not even on.

Scott · #4 02-05-2010, 12:49 PM

Thanks Luc. Seafoam is one thing I haven't tried yet...I guess if I've been running on a bum O2 sensor and it's been running rich it could be dirty by now. Worth a shot I guess.

RangerLuc · #5 02-05-2010, 12:53 PM

yep. theres a how to somewhere around here on how to use it but i put it into my brake booster line and you can also put it in your oil, and gas tank and it really made a difference

Scott · #6 02-06-2010, 12:37 PM

Just had codes pulled last night -

P1506 (IAC overspeed error) which points to a bad IAC valve or a vac leak. IAC has been replaced, so for the 1000th time, I will check for vac leaks.

P1131 (lack of bank 1 o2 transitions - lean) which points to vac leak or fuel injector problem.

Maybe I should just hit the craps table instead.

00bamaranger · #7 02-19-2010, 08:28 PM

mine was doing the same thing a while back, i replaced the IAC valve and it seemed to fix it but then it started back again a few weeks later. i took the valve back and got another one and it didnt fix it. i finally traced it down to a vac leak and i havent had any problems out of it for a while

RangerIDLESat45mphWTF · #8 03-05-2010, 07:52 PM

Having pretty much the same problem, same code# p1506, havent replaced anything yet but on a budget and looking for answers also.
I have a 97 ranger that will idle like its at full throttle. It doesnt do it all of the time, but it has been doing it alot lately. If you push the clutch in and put it in nuetral while driving the rpm will rev WAY UP like its at full throttle. Lubed the cable and mech on top of throttle body, cleaned the IAC valve with carb cleaner. The pedal, cable and mech operates smoothly. wondering if there was an EASY WAY to test the IAC valve ?Any suggestions would be great. I dont have much money to work with and it is my only vehicle as of right now. I have read other people replacing alot of things and still having the problem and I cant really afford to buy a bunch of new parts that dont do anything.

Scott · #9 03-06-2010, 09:28 AM

Haha I'm one of those guys who's been buying a bunch of parts...not that I can really afford it. Every time I get frustrated I just keep telling myself "It's either this or a car payment". Anyways p1506 is defined as an IAC overspeed error, meaning the ecm has detected an engine speed greater than desired. Probable causes: engine vacuum leak or failed IAC valve.

To test the IAC:
1. Turn ignition off if its not already
2. Disconnect wiring harness from IAC
3. Use an ohmmeter to measure the resistance between the terminals
4. Resistance should be between 7-13 ohms. If not, clean and try again. If results are the same, you need a new one (about $50).

I have also heard of unbolting the IAC from the throttle body, leaving the wiring harness hooked up, and turning the ignition to the on position without starting the engine. You should see the valve inside the IAC move to the fully open position. This way you can visually confirm that it is working.

Since my last post, I've replaced my IAC and TPS sensors and I still have the same problems. I did find a nice vac leak on one of the lines going from the vac manifold to the intake, but I can still hear another one. I'm about to take the intake apart and check all lines and gaskets for the leak. I also have a complete exhaust to throw on. If its not the IAC i would bet that you have a vac leak somewhere. All this vacuum assisted - sensor dependant - computer controlled CRAP makes me not want to buy a new vehicle. My next one will be something carburated with NO COMPUTER!

linda · #10 08-18-2010, 04:22 PM

I have had my truck at the shop for 8 days now. They tell me I have water getting in the block. Have to start my truck let it run for 15 minutes then I can drive all day long no problem. Turn truck off go to work come out same thing coughs real bad have to sit for 15 min. or drive 5 mph for 6 or 7 miles. They tell me to trash my truck. Its a 94 Ford Ranger 4.0 6 cly. Red . Love my truck dont want to get rid of it. Really good shape inside and out. Any suggestions out there love to hear some thanks. Trying to hold on to my boy.

linda · #11 08-18-2010, 04:37 PM

what is seafoam

Scott · #12 08-20-2010, 04:04 PM

It sounds like you have a gasket leak that goes away once the engine warms up (and metal parts expand). I'd tear it apart and replace all gaskets from the heads up. It's a PIA, but if you do it yourself it won't cost much in parts.