98 engine woes...

98 4.0 ax15 np231

Last Saturday I got into got in the dd and left for class. Noticed right away that the idle seemed high. Couple of blips on the throttle it settled right down. Get to the first stop light, clutch in, idle drops to near zero, then stumbles up to 900. This happens at the next three stops, then goes away.
Today I am crising on the highway at 80 with the cruise control on. With out warning engine bucks and cruise shuts off. I throttle, engine begins to buck more, and violently. Clutch in, idle appears to be somewhere in the 1500 range. Clutch out, throttle, smooth going for about five miles and then more bucking. Cruise is lit on the dash, but will not engage. I turn on vent controls, all are functioning properly.
When I got home I pulled the throttle body and cleaned it thoroughly. No affect.
Tomorrow I plan on hooking a pump to the vac system to search for leaks, am also planning on finding my multimeter, so I can start testing sensors...

- are these two problems related?

I will post up more as I test...

Check battery voltage first. About 12.6 sitting. Aprox 13.6-14.3 running. The computer likes correct voltage to run properly.

Battery tests out at 12.93 sitting, 14.01 charging
TPS tests as .81 closed 4.05 WOT

Going back out to check more...

edit;

Just checked the vac system using an old nebulizer. System took pressure, found no leaks in vac lines

Just took the rig out for a hard drive. Throttle response was smooth as ever, cruise engaged with no problems. I am leaning at this point that the values on the TPS are off enough that I will go ahead and replace it. My theory is that the lower values on the TPS are translating into the cruise not wanting to engage yesterday?

Checked the TPS with an analog ohmmeter?

I used the digital that I have. I know that it is recomended to use analog, but I do not have one available to me at this time.

I had this sort of thing happen to me. The solenoid which adjusts idle air bleed was stripped and could not adjust consistently. It's like a $5 part.
Located on the TB.

WB9YZU would you have a part number on that, and or a description of where it is on the throttle body?

http://www.napaonline.com/Catalog/CatalogItemDetail.aspx?R=CRB229571_0169845657

"Idle Air Control Valve"

It's a slug on a worm gear with a tiny motor on the other end. 2 screws, no adjustment.

When you get the new part, you will be tempted to play with it. I recommend not doing that, just put it on and the ECU will do the rest.

I cussed, I swore, I finally figured it out one day after meditating long and hard about the problem.
I got one from the JY, it was bad too :flame:

Symptoms were:
Low idle, no idle.
Rough idle (Sometime smelly).
Sometimes severe drivability issues.
The code it would sometimes throw indicated that it was running rich and had exceeded the ability to adjust itself.

Operation is simple.
There is a motor with a screw on it. The slug has mating threads and a slot in it. There is a grove in the bore to catch the slot. When the motor turns, the slug moved in or out depending on motor direction. This thing is in constant action trying to compensate for your throttle input.

You could take yours, unplug it (You will get a code saying it's missing). Adjust it for best idle (remove, spin slug, install -repeat) just to see if that's your problem. I did that and I knew right away that I got the right part in my sights.

Since I just looked up the part. Well, I was wrong, it's just a $89 part at NAPA. I'm sure you can get it cheaper someplace else.

Gotcha, I wasn't comprehending what you were talking about the first time. I did have an inkling that the IAC could be going, but I hate to throw money at parts.

-Could my original symptoms be caused by the numbers I pulled from the TPS, ie; stumblebuck and loss of cruise?

-Any votes that it is the IAC, any way to test besides pulling the plug and driving without?

I don't know that it means anything, but when my TPS failed, it caused bucking/hesitation under constant throttle, but not when accelerating or decelerating. It didn't trip the CEL, and cruise still worked. I wouldn't think the IAC would have much of an effect when the throttle is open though, or would it?

Hubs, my thought exactly. I know when the IAC died on my 88 it caused crummy idle, but it did not affect my acceleration/ constant throttle. Hopefully someone can shed some light if my tolerances on the TPS are far enough out of the range that they may have been affecting how the cruise servo read what the throttle was doing/ not doing to shut itself down?

xjwisconsin said:

Hubs, my thought exactly. I know when the IAC died on my 88 it caused crummy idle, but it did not affect my acceleration/ constant throttle. Hopefully someone can shed some light if my tolerances on the TPS are far enough out of the range that they may have been affecting how the cruise servo read what the throttle was doing/ not doing to shut itself down?

Click to expand...

TPS Testing:

The TPS can be tested with a digital voltmeter.

The center terminal of the TPS is the output terminal.
With the ignition key in the ON position, check the
TPS output voltage at the center terminal wire of the
connector. Check this at idle (throttle plate closed)
and at Wide Open Throttle (WOT). At idle, TPS output
voltage should be greater than .26 volts but less
than .95 volts. At wide open throttle, TPS output
voltage must be less than 4.49 volts. The output voltage
should increase gradually as the throttle plate is
slowly opened from idle to WOT.