Hey Gang. I borrowed my dads truck. The idle is real high on this thing then it settles. I changed the IAC and checked the TPS. the TPS seems fine as I get 5 volts there and .36 volts closed throttle and the value climbs steady as I open the throttle and values dowm smooth when I close.
When I drive the idle goes to 2000 rpm. if I put the clutch in the Rpms will remain high and when I come to a full stop with the clutch in I can watch the RPMS drop to idle at 900. Sometimes the truck with speed up if i put in a higher gear because of the high idle. its like built in cruise control. yet i dont have that option lol.

Im curious to know if there is a Speed sensor on trucks equipped with a manual 5 speed trans 3.9ltr. Im thinking it my be my problem. only 196,000 km on it, and its a 1996.

Thanks..

 

I believe that the sensor is on top of the rear diff if its 2wd. If its 4wd then I think its on the t-case. I could be wrong though.

 

Its 2wd and standard. I was under the impression that only automatic vehicles had speed sensors. Ill have a look later and see if there is a sensor there for i cant figure out what other problem would cause my high idle.

 

1. Check to make sure intake isn't leaking air at the rear wall.
2. Vacuum test. Post numbers at idle.
3. Make sure the throttle blades are closing. Carbon will kill the seal.
4. TPS voltage is low. I dont remember off hand for the v6 but I think tps codes the MIL high at .79. And you want it as close to .78 as possible. I'll look tomorrow or low level mil indication. You may have a flat spot in the windings. Hook up your volt meter and slowly crack the blades from idle to wot. You are looking for a spot where the tps fluctuates irregularly.

 

Thanks for the reply. I cleaned the Carb out with Carb Cleaner. you can almost see your self in the ports so everything is good there. I will look at my voltage results again when I get home from work and do a vacuum test. I checked for intake leaks using a propane tank for plumbing. I use to do this for my chevys and locate leaks there when i hear the rpm rise i take the propane away and repeat.

On the dodge is there a preferred area to test with the vacuum gauge?

TPS, I read an Thread on here about testing it from closed to WOT. I followed it and got the same results. A very steady increase and decrease.

Im not sure what the black box is on the front of the carb. I see wires going to it and and on the bottom a vacuum line. I went to the scrap yard and took every IAC TPS and that black box off. Whish I took the wires off of em as well since the darn plugs are not the same as mine except for the IAC.

 

The black box is MAP sensor. Post a pic if you need something identified.

MAP will trip a mil if bad. So you are probably all set. Check that area or leaks too. And the sides of the TB and the entire intake area.

Check vacuum constant below the throttle plates.

Is a mil on or is one pending? Autozone and some other vendors will pull a pending code for you.

And lastly, don't discount normal consumables for wear and tear. Cap and rotor, plug wires ( and plugs and gap), air restrictions, non closing t-stat or one two low,

Idle is set at an rpm from factory as well as part throttle parameters. The system learns slightly and she WILL surge if trying to calibrate. You were correct in assuming IAC because the surge or hunt is usually the IAC trying to find constant rpm. Good call. However, the computer will bump rpm to prevent a stall and then let the IAC reset.

Whenever you change a part in the sensor chain, reset the pcm to go back to base tune parameters. If you dont have a tool for it, simply remove the positive and negative cables from the battery and hold the two together for a 30 second count. This will dump learned memory ( and pending codes). So I like to check the cache for pending data first. The computer will be at base tune once memory is erased.

 

Going along with the vacuum leaks, take a look down the throttle body with the plates wide open. The plate that you will see is the bottom of the plenum. If you see any oil at all, the seal is probably broken and this could be causing part of your problem.

 

Well Im about to rip my hair out. I put a vacuum gauge on it and hooked it up below the carb, its maintains a good vacuum and goes to zero when i peg the throttle. I sprayed carb cleaner in and around all the gaskets hose etc and no sign of rpm increase. No oil in the bottom of the plenum. I bought a code reader a Maxiscan MS300 and a few codes came up. i cleared the codes and have been running the truck over the weekend and no check engine light, however, i still have a very high idle untill the truck comes to a stop..

Im wondering if it has anything todo with the ecm.

 

1. It the throttle returning to zero at idle. Disconnect the cable. Restart.
2. What was the vacuum at idle? What is good? Post the number.
3. What were the codes you pulled? If you didn't write them down, drive it. It will code again.
4. Doubtful that its pcm. You could be correct but Im not a big fan of throwing parts at the diagnostic phase. TPS, IAC, MAP and the Temp sensor all have voltage correct?
3. Did you at anytime rotate the distributer? Has it moved?

 

Some Of the codes I had to ignore at the moment because I know they were tripped as I unplugged things like the map sensor etc.

I was getting a constant 20inchs of vacuum or mercury as some refer to it. When I unplugged the map sensor it would give me 18 inches and then some times bounce simulating a bad valve or cylinder but it was all god when i put it back. .

As for the distributer, I tried to do the timing a last year when I did a tuneup and realized that this was not an option. I brought the motor to TDC to ensure the base timing was correct since my buddy thought hed help and remove all the wires at the cap not knowing i was going into the garage to grab a marker to label them. There is a slot on the distributor and the block and this would not allow you to turn the distributor any ways.

 

Pull the IAT sensor and clean it. Re install.
Check for voltage at the temp senor.
What T stat you have? 165 or 180?
Spray carb cleaner at the shaft bushings of the throttle body. Check for leak.

Then run it. We need codes...

 

Tnaks again for working with me to resolve this issue. Oddly no codes still. Ive driven overr 400km since i cleared the codes. Still uses gas like a battle ship. 40 ltrs get me about 218km.. Ill check the temp sensor etc as you suggested. Im off to the scrap yard today. There is a 50.00 all you can carry weekend coming up. so this is why i toss parts at it like the ecm etc

Is there any thing you guys are looking for. The more parts I carry out for 50.00 the better. Let me know I can ship them from Oshawa Ontario Canada if there is something you guys are looking for..

Ill check those things tonight and post results..

 

Does it have any guts or is it out of power?

You may have clogged cats? Which will ramp up idle speed to stay running. The t stat questioned I asked is because if the vehicle is over cooled it will stay in "warm up mode" and not transition to run. The computer works different run cycles for emissions.

Without you testing the sensors that tell the pcm what to do changing parts wont net any difference.

I hope your parts hunt goes well.

 

Now thats a good point.. Ill disconnect the cat and have a look at that. It has some ***** but not a whole lot. does not accel fast as it should but has a wee bit of *****. I figured it was a v6 dodge thing compared to my exploder.

I swapped out the temp stats still nothing and I have a 180F thermom in there and according to my inferred thermom its operating in the right range.

There were quite a few donars in the yard today so Ill be back for some bigger things on the weekend. I just grabbed sensors and tossed em in my tool bag.

Engine code came up today . P0138 referring to a high voltage on the o2 sensor and the a p1294 p no idea what that is.. so not sure if this will cause a high idle. Just remember I dont have a high idle issue when stopped. it idles fine. but when Im rolling a bit it goes high. 2-3k when rolling at any speed and then 900rpm when stopped. soon as i come to a complete stop the idle will drop from 3000 to 900.. this is what confuses the hell out of me.

 

Code 1: Your down stream o2 sensor is reading all the fuel / oil you are putting in that thing. Do NOT change the sensor. Its only a tattle tail light. There is no feed back to fuel or spark trims. It only reads differential data. If signal is same as front o2 it codes. If its outside parameters high or low, it codes. Pretty basic.

Code 2. Target idle ( 720 rpm I think for yours ) can not be achieved.

BINGO!!

I bet you are using oil. The intake manifold is a two piece design. There are the typical head to intake gaskets and then a plenum to valley gasket. When it fails you suck valley oil into the combustion chamber. The design was HORRIBLE. So many failed that they changed they ordered reflash for the computers and took timing out. It was called the death flash.

Change the intake plenum gasket. My guess is that is your leak.

 

http://www.hughesengines.com/Index/p...3&partid=10655

This is what it looks like.

 

PK1, I have not noticed alot of oil consumption, Ill pull a plug or two and see how they look, if I recall from auto class in college, anything other than a tan brown will tell a tail. to rich, to lean, water, or oil. as for the plenum That looks like the same contour. I thought that when I pulled the PVC valve and plugged it and then went to the other valve cover and removed the tube to the breather and plugged that at the same time, waited 5 seconds or so and then released, On release if it poped and blew out it meant that it was building proper pressure and ruled out that i had an internal vacuum leak/ bad seal.

 

The plugs wont oil up. The computer is working OT to keep AF at 14.7. It has no idea that there is oil in the system. It just adds /removes fuel and air as needed. Your indicator was the second OBD code.

When guys add aftermarket valve covers or remove valve cover baffles from inside the stock ones the PCV sucks oil of the top of the rocker below it. Some even tap the side in the back to hide the pcv and that is worse. I asked to make sure you werent consuming oil from another location.

College taught you well on the reading of plugs. I wish my college had those classes back in the day. Just remember your OBD system wasnt coding a misfire from the primary upstream o2 sensor so its happy with the AFR that its seeing. the plugs won't scream problem to you. It could code any second but Im afraid your cats will be contaminated by then so I wouldn't wait. Gasket is 20 bucks. Cats are $$$$!

 

Well they actually taught us that in grade 9 auto. When we use to clean and re-gap plugs rather than toss them out. We had an champion spark plug cleaner and tester. it used compressed air and sand to clean the plug and had another chamber so we good analyze the spark with a powered coil. College did this again and that was over 18 years ago for me.

I took the cat off of the truck. its spotless inside. I can see right through all the "honey cobes". Im going to replace that gasket after i put the exhaust back on. Im surprised how good this thing is.

 

Well I removed the intake and plenum gasket. They were fine. had a perfect seal all around so I had to scrape it all off and put in the new ones. Motor will still idle fine when in neutral or clutch in, but if the truck is rolling at all the rpm will go to 2000 sometimes 3000 untill i come to a complete stop. This is the weirdest thing and only codes I get is a high voltage on the O2 sensor as mentioned and sometimes a miss fire code. but after i clear the codes it make take 3-4 days before the check engine light comes on and gives me a code.

hmm..

 

The system codes short ( pending codes) and long term (actual). It usually takes 3 run hours to codes the latter.

Their is no misfire sensor. Its an abnormal fuel mixture seen by the upstream 02.

Rotate injector, plug and wire from offending cylinder and if the misfire follows it to it's new home you have your culpret.
3. If it doesn't follow the swap in communication between the mpfi or an defective sensor / vacuum leak. Reset fuel sync to +0. Recheck vacuum, and get your sensor voltages.

 

PK1, Thanks again for sticking with me and adding your knowledge and wisdom. I will give that a try. Actually that was pretty smart and I feel like an *** for not thinking of swapping it LOL>

Ill let you know..

As for the codes. I didnt know that the brain didnt trigger the CEL right away. I thought once it detected a few of the same errors it went off. Explains why it takes 5 hrs of driving before it triggers.