Engine problems, possible TPS or IAC?

Hey guys, once again my Jeep is emptying my pockets...
Saturday, I was driving home from work (40km trip) gas light was on, forgot my wallet. So I had to go home and get it

1. Shut Jeep off, go inside grab wallet, come back out, Jeep starts as normal

Get to the gas station (1km away). Fill it right to the tits.

2. Attempt to start Jeep all it does it crank crank crank. I try again but put my foot to the floor and she fired up. So I thought damn this is really wierd.

3. Start driving and notice the Jeep as acting as if the throttle is sticking. Cruising along at 70kmh, it would continue to hum around at 1500rpm without my foot on the gas.

Get to my buddies house, shuts off as normal, sits for about 45mins before we go back out, same problem starting, have to have my foot to the floor. Notice the same symtoms while driving as well (like I have cruise control or something)

Come back to buddies shut it off, 4 hours later, it starts up perfectly fine, I drive home don't notice anything funny. Shut it off and go to bed.

4. Next morning I start it up, no problem starting, 15mins into my commute I am getting the "cruise control" symptoms again...reach distination, shut Jeep off, attempt to start it, I have to have my throttle pegged to the floor for it to start. Runs like bag of shit at this point. Drive home and park it, 8 hours later I started it and it started fine as normal, took it for a drive again and symptoms appeared once again.

Now today (Monday), stone cold started perfect. As soon as I took off the "cruise control" feeling came back. I drove to school where I am now, in the parking lot I shut it off and tried to start it, wouldn't start unless the throttle was to the floor. I looked at my idle speed it was around 1100rpm. In "P" I reved the engine to 1500rpm, the rpms stuck there for 5-7 secs after I released the throttle, revved to 2500rpm same results, stuck there for about 5-7 secs

I let it come back down to "idle" which is still around 1100rpm put the Jeep into "D" the revs drop to 600rpm, little bit of engine hesitation then they go up to 800rpm (normal idle), same deal with reverse.

Jeep is not throwing codes...
1996 4.0L/AW4/NP242

Did an oil change 300km's ago with Amsoil and K&N oil filter. Replaced Oil Pressure sensor at the same time because I broke it. I have not had any mechanical problems until now (this past Saturday Nov 18th)

Searched for TPS problems but info that I found was concerning the old Renix engines. I don't know what an IAC is, but within my searches it seems to be a related problem with poor starts, funny idling etc. I need your help guys and gals, I am stumped...:banghead:

Do you have cruise control? it could have to do with it if it is not working properly

nope, no cruise control :laugh:

IAC stands for idle air control,on my 88 I had to adjust the idle because it would not work even after I replaced it,but,the 88 was a wore out junker.I hate to tell you this but the easy way to find out is with a scan tool,otherwise you are really just guessing.But if you want to try something,get the engine started and idleing,unhook the IAC and drive it,it might stall on you until you get the hang of driving with no idle compensation but you will see if it cruises away on you or not.What does it run like after the hard start?OK or some stumbling for a second or two?:explosion

on a fuel injected engine holding the gas pedal to the floor shouldn't affect anything...weird. Does your gas pedal feel sticky at all? the cruise control feeling could be a sticking throttle cable or throttle body. i'd suggest getting some throttle body cleaner and just pull the intake hose and clean out the throttle body and as much in that area as you can. I would think a TPS problem would be more involved with trans shifting and low/high idle problems but not starting up problems. i could definately be wrong on that though.

Certainly sounds like something is hanging up. Checkout and clean the throttlebody and look at the throttle cable. You might also have a bad vacumn leak somewhere. Oh and holding the throttle all the way down while you crank is how you are supposed to clear a flooded engine, it tells the computer to shutoff the injectors.

If I don't get a "hard start" it usually will drive fine for a couple of minutes before the cruise control feeling kicks in. Update while driving home, if I stab the throttle while I am already rolling (pedal to the metal), the engine starts to stall out, the rpms drop to 1000 then they spike and I take off.assgas:
Immediately had symptoms while coming back from school because I had a hard start.

update: tried to get the code it threw on the scanner. Nothing came up on the scanner?! So we cleared the codes with the scanner and are waiting for another check eng light to come on.
For shits and giggles we disconnected the TPS sensor and the problems only multipled. Disconnecting the TPS however did throw a code so maybe it's working 100% still and something else is on it's way out? :dunno:

New TPS sensor is $35
IAC Motor is $60
Haven't bought them yet, just for reference...

Is there anyway I can "force" the code to show up? Really stuck here guys. I need my junk to survive my 120km weekend commute once again at the end of the week.

Muddygp, I hate to correct you but holding the pedal to the floor is sensed by the computer at start up as a flooded condition and causes the fuel flow to the injectors to be stopped.

My guess is that by running the tank down so low and then filling it, you stirred up some crap in the tank. Fuel filters do a poor job. I bet you have some crud stuck in an injector. My recommendation is to change the fuel filter unless it is fairly new and then add a can of BG44k to the fuel tank and run it for a few days. It will remove any buildup in the injectors and remove carbon from the combustion chamber and valves. I run a can every 10k miles in every vehicle I own. It is a good thing to do anyway so it isn't a waste of $$$ if it doesn't happen to clear up this problem.

Only then would I spend any money on new parts. I hate spending money on parts that don't fix the problem. Do a search on here, you can find the instructions on how to check out a TPS with a simple volt/ohmmeter.

Muddygp, I hate to correct you but holding the pedal to the floor is sensed by the computer at start up as a flooded condition and causes the fuel flow to the injectors to be stopped.

My guess is that by running the tank down so low and then filling it, you stirred up some crap in the tank. Fuel filters do a poor job. I bet you have some crud stuck in an injector. My recommendation is to change the fuel filter unless it is fairly new and then add a can of BG44k to the fuel tank and run it for a few days. It will remove any buildup in the injectors and remove carbon from the combustion chamber and valves. I run a can every 10k miles in every vehicle I own. It is a good thing to do anyway so it isn't a waste of $$$ if it doesn't happen to clear up this problem.

Only then would I spend any money on new parts. I hate spending money on parts that don't fix the problem. Do a search on here, you can find the instructions on how to check out a TPS with a simple volt/ohmmeter.

If the engine has over 40k miles on it, the BG44K will most likely help your fuel mileage considerably.

I have been having the EXACT same problem you described, except for the hesitations/stumbling when running. Mine has a coolant leak and i think the low coolant was messing with my IAC, it went away when i added coolant and drove a bit.

You might want to suspect the IAC still, and possibly fuel filter

Hey, this sounds very familier. I had a similar problem. After 6 years of ownership mine stalled one morning for the first time ever and would not restart untill I held the throtle open. I noticed the idle rpm holding a few revs higher than normal as well. When it stalled a second time a week or so later I flashed the codes and came up with a (15) coolent sensor low temp. I unsnaped the conector on the sensor at the thermostat housing and reconected it. When I restarted the engine it was normal start, normal idle. I wish it always worked that way.

Two things low engine temp causes: Rich fuel mixture and high idle. This would be normal effect when starting a cold engine; one that has set long enough to drop to ambient temperature. If the temp sensor was telling the ECM that a warm/hot engine was "cold", the ECM will enrichin the mixture and increase idle speed. This kind of fits with the experience of Jason-RJR and Alpine-XJer. J-roc used the "clear flood" mode when he successfully restarted his warm engine (throttle wide-open), and observed unusually high throttle while cruising at low speed. Maybe worth checking out his engine temp sensor and associated wiring.

xjbubba said:

Two things low engine temp causes: Rich fuel mixture and high idle. This would be normal effect when starting a cold engine; one that has set long enough to drop to ambient temperature. If the temp sensor was telling the ECM that a warm/hot engine was "cold", the ECM will enrichin the mixture and increase idle speed. This kind of fits with the experience of Jason-RJR and Alpine-XJer. J-roc used the "clear flood" mode when he successfully restarted his warm engine (throttle wide-open), and observed unusually high throttle while cruising at low speed. Maybe worth checking out his engine temp sensor and associated wiring.

Click to expand...

makes sense.

last night i found the waterpump leaking, and when i refilled the coolant and got the air worked out, all the idle issue and hard start problems went away.

now i just gotta fix a waterpump.

Well, for starts here is what I have done to my Jeep engine wise to get it on the road:

New Water Pump
New Rad Fluid and Water Wetter
New T-Stat (195)
New Lower Rad Hose
(1000km's)

Oil Change and filter 300km ago, along with new oil pressure sensor because I snapped mine in the process

Was getting warm operating temps with the leaky water pump, now it runs half way between the first marker and 210. So it is a little cold, but it has been running like that the past month and a bit so it's odd that problems didn't occur sooner (if it is infact the temp sensor)

Stone cold it started perfectly fine this morning started without any throttle, but as soon as I started moving I had the cruise control feeling once again...went to the gas station today and put some 91 in it, to see if I also got a shitt batch of gas. No change in how it's been starting when warm (needed to start full throttle)

How can I test the temp sensor?

Edit: Since this problem started I have ran injector cleaner through it, with no noticeable difference. At the same time I bought a fuel filter but I have not currently installed it.I don't want to be buying a bunch of junk and not have the problem fixed you know?

How can I get teh low temp sensor to throw a code?

Check the temp sensor with a scan tool,my 88 had a leaky injecter ,thats why I had to floor it to start,no money to fix at the time.Also check the connection at the sensor as has been mentioned before.I hate to say this but if you know a good mechanic,spend the money,you will be ahead of the game in the end because he can see and drive it.:rof:

Appreciate the kind words, the only ones I know are all cripples and crooked. This would be so much easier if the damn engine would throw a code.

Like I said before, I had the TPS disconnected last night, check engine was on, it finally cleared itself today almost 20 hours later... I think I can rule my TPS as a possible culprit.

Installing a new fuel filter for shits and giggles tonight to see if it makes any difference. When I first bought the thing, the PO left it bone dry for me, and I filled it up full and this problem didn't happen :dunno:

It's a Jeep thing :looser:
Scored a IAC so we are going to install that as well, maybe I'll shoot some throttle body cleaner down the intake tonight as well.

Next will be the coolant sensor and then the TPS

Probably the throttle positioning sensor (TPS). You can try cleaning it and then squirting some WD40 on the part that moves....

Another tip would be to really clean the Idle Air Controller (IAC). It is the sensor to the left of the throttle positioning sensor, if you are standing on the front of the vehicle.

After cleaning the IAC, squirt some WD40 inside where the spring is. Sometimes the lube on the spring hardens up, and the WD40 loosens it up nicely and it will work normally again.

Good luck!

IAC was extremely gummed up, cleaned it out and sprayed all the connections down. Works mint now, bought a TPS and IAC, going to keep em as spares. Thanks !!

I'd just take them back and get the $$. If everything works fine in that area, props. You can always buy them later. I'd rather have the cash!

Glad to hear it's back to normal now!