1990 quick questions....fast idle & fuel pump ???

Hi there,

I have a few questions about my 1990 Jeep with 4.o, since I noticed recently:

1. Sometimes, when I start the jeep,the motor revs to 2000 and stays there. After driving it, it will drop to regular idle. Almost like the throttle cable is stuck, but I can't step on the peddle to make any real changes. Any thoughts?

2. My fuel pump is starting to make noise, you know the hum. How soon do I need to change it? Can I use a aftermarket pump, or should I use a Jeep part? How hard is it to change at home???

Thanks in advance!
Bob

If you do not have a multi meter, volt/ohm meter, get one. They cost anywherre from $3 to $20. My local Harbor Freight had them on sale for $2.97 the other day. If you can find an aford one, the old analog style meters are best for sensor testing on Jeeps.

Start out by testing all the sensor ground wires for a maximum resistance of 1 ohm, from the wire at each sensor to the battery negative post ground. Good chance your idle problem is poor, dirty, oxidised ground connection.

Look up and read the "Renix Files" thread and links in that thread for individual sensor testing details for sensors that affect idle.

Noisy fuel pump is not necessarily a sign of impending failure. Mine is noisy until the noise is drawned out by the engine running and has been for 3 years, 25,000 miles. For more details on fuel pump replacement search this forum for "fuel pump" threads, lots of to threads already here.

cherokee4fun said:

Hi there,

I have a few questions about my 1990 Jeep with 4.o, since I noticed recently:

1. Sometimes, when I start the jeep,the motor revs to 2000 and stays there. After driving it, it will drop to regular idle. Almost like the throttle cable is stuck, but I can't step on the peddle to make any real changes. Any thoughts?

2. My fuel pump is starting to make noise, you know the hum. How soon do I need to change it? Can I use a aftermarket pump, or should I use a Jeep part? How hard is it to change at home???

Thanks in advance!
Bob

Click to expand...

Thanks for the response,
SO I do have a meter, thanks for input, I will check that. When should I get worry about the fuel pump? Do they get louder over time?

Thanks in advance,
Bob:rof:

Sometimes they get louder and sometimes they stop and sometimes they will run loud for several years....you just don't know. What I do when I suspect something is about to go bad is start looking online for a reasonably priced quality replacement (sometimes even used if its cheap enough). Then I read over how to fix the problem and hold on to the part so if it does happen to go bad at 8:30 on a Sunday night, you already have the part and can still make it to work Monday morning.

Aw shucks, that just takes all the fun out of :bawl:wondering if you're gonna get to take Monday off to fix your jeep.

Gotta have some suspense in your life!

Jess said:

Sometimes they get louder and sometimes they stop and sometimes they will run loud for several years....you just don't know. What I do when I suspect something is about to go bad is start looking online for a reasonably priced quality replacement (sometimes even used if its cheap enough). Then I read over how to fix the problem and hold on to the part so if it does happen to go bad at 8:30 on a Sunday night, you already have the part and can still make it to work Monday morning.

Click to expand...

When mine is doing the 2000 RPM idle thing, the first thing I try is to shut off the engine and start it again. The latch relay reparks the IAC. And after a restart, if it idles OK, I remove and clean the IAC. I spray off all the collected junk on the piston/stopper, clean the seat and let a few drops of oil run behind the piston. A regular synthetic motor oil seems to work best for lubricating the piston. Behind the piston there is a slotted rod (or screw shaft) that is driven by a small motor. This moves the piston in and out. The screw shaft is lubed with grease, junk gets in there and makes the grease thick and possibly gritty.
Sticky IAC's are fairly common. Low battery voltage or like Ecomike mentioned poor grounds add to the problem. The IAC seems to need just about all the voltage it can get to work well.
Replacing the IAC is an option, but they are expensive. I clean mine twice a year and since getting into t his routine, have had few problems with either my 88 or my 96. Both of which have had sticky IAC's in the past.
If I'm in a hurry and don't have time to remove the IAC, I sometimes spray a good penetrating oil down the IAC orifice, while the motor is running. I work the throttle a little, so the IAC piston moves also. This often unsticks the IAC, at least for the short term.
High idle can be caused by various things, everything from a vacuum leak to a faulty TPS. But the IAC is the first thing I check, especially if the problem seems to be temperature related or goes away after a restart or two.

Ecomike said:

Aw shucks, that just takes all the fun out of :bawl:wondering if you're gonna get to take Monday off to fix your jeep.

Gotta have some suspense in your life!

Click to expand...

My Jeep always seems to know when I've forgotten my cell phone and picks the exact moment to act up.

Nice avitar!

So you've joined the paying members club!

8Mud said:

My Jeep always seems to know when I've forgotten my cell phone and picks the exact moment to act up.

Click to expand...

The fast idle thing comes and goes. I hate to ask, but what & where is the IAC?

Thanks,
Bob:spin1:

cherokee4fun said:

The fast idle thing comes and goes. I hate to ask, but what & where is the IAC?

Thanks,
Bob:spin1:

Click to expand...

IAC = Idle Air Control

You have a 90, so its got the RENIX computer system. To you that means that if you look at a 91+ engine its going to look slightly different from yours especially around the throttle body.

If your looking at the throttle body from the front of the engine, you have two "sensors" that have electrical wires attached to them. The one on the right has a control arm that gets moved when you open and close the throttle plate either with your hand or with the accelerator. That is your TPS (Throttle Position Sensor).
The one on your left just looks round. That one is your IAC.

Im going to bet you have one of two problems that you should check first.
One is check for vacuum leaks. Use some soapy water in a spray bottle and spray (on mist) all the vacuum lines that you can see while the engine is running. Also listen for a hiss, like a vacuum line is disconnected or broken. As your spraying, if you hear the engines idle change (or a hiss anywhere), you found where there is a vacuum leak. Fix it, and continue searching.
The other thing that most likely is causing it is a poor ground. Check all the ground wires you can. Clean them up, and or replace your main ground wire to your battery. The most important one related to the problem your having is the braided one that goes from the firewall near your MAP sensor (Black sensor on the firewall with a vacuum line from it going to your throttle body) to the engine. It goes on the head bolt at the rear of the engine right next to the firewall. Its a pain to get too and clean up. I couldnt get mine loose because of clearance. You may be able too. If you can, toss that braided thing, and get something like 4 gauge and bigger and replace it. If your like me and cant get to it enough to get it off, then spray the hell out if it with WD-40, or brake cleaner (Preferrably WD-40). Then where it mounts on the firewall, run a 4 gauge or bigger battery cable from it to the ground where the battery connects to the block on the other side (Hard to miss).

I hope that helps you out.

I had no problem removing the braided wire between the head and the firewall on mine. I just scotchbrite cleaned the flat contact areas on the firewall, engine head and the flat wire connectors at each end. Got clean bare metal exposed and re-attached the same old braided wire on mine. It's not the wire that is the problem, it is the dissimilar metals galvanic corrosion that sets up over the years between the contacts on the firewall, engine head and the connectors on the end of the wire. The ground on the engine block near the oil dip stick and oil filter adapter is another spot where the end connectors at the end of the ground wire and engine block surface need to be scrubbed clean to bare mare metal and reconnected.