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High Throttle at startup - TPS?

B

box_goblin

NAXJA Forum User
Location
Seattle, WA
Intermittent throttle issue that has gotten progressively less intermittent.

I wanted to check a few things with some folks with more experience before part swapping, here goes:

90 XJ, 4.0L, AX-15
Throttle (more often than not) spikes to 2-3K right after startup. It does not normally come down on it's own, but will more often than not go back to normal (~750) if I drive and force the engine to lug down using the brake while in 1st.

I know that the TPS is the questionable part here, so I did test mine per the chart in the FSM.
Input Voltage: 5.02 V
Output with Throttle off was showing 0.65V
I adjusted the TPS to set output V up to 0.85 per the manual's specs and fired up the engine.

Engine went right up to 2500 RPM
Unplugging the TPS revs the engine back down to a normal idle (why does it do that, anyhow?). Plugging it back in immediately after the disconnect and the idle remains normal; it does not spike back up. The temporary interuption to the TPS seems to reset it some how?

So, the question really is, does this indicate that the TPS is at fault here? Or could it be something else? Thanks in advance!
 
Something else. Take out the idle speed control motor and clean it where it seats in the throttle body and also clean the throttle body out while your there.
 
Not likely to be the IAC based on the test he did. So they are "Not the droids you are looking for".;)

A stuck IAC would not drop and hold proper idle right after disconnecting then keep that correct idle after reconnecting the TPS.

You need to test the MAT and CTS for proper resistance at 3 temperatures.

One major, very common possibility is a poor TPS ground that is changing resistance and leading to a variable idle , loose, dirty ground. Test the TPS ground wire on the ECU side!!! Should be less than 1 ohm to the battery negative post. Also test the TPS idle voltage with the elevated idle to see if the TPS has gotten sloppy (dirty, noisy, inconsistent output) internally.

Clean all the battery and vehicle main ground connections. You want bare metal on both faces of the connection.

I had to run to new TPS sensor ground wires directly to the battery on my 87.
 
There is a relay that holds power on the computer for a few seconds after you turn off the key to give it time to reset the IAC back to a cold start idle position. If that relay is not working the power is lost before the IAC can be reset and when you first start up, it is too open and the rpms will spike. IIRC it is called the B+ Latch Relay.

It may also be a sticky IAC stepper motor. Cleaning may help. It never hurts as long as you do it correctly. Remember that if you pull the relay, make sure the battery cable is disconnected or the IAC could be ruined.
 
Ecomike said:
Not likely to be the IAC based on the test he did. So they are "Not the droids you are looking for".;)

A stuck IAC would not drop and hold proper idle right after disconnecting then keep that correct idle after reconnecting the TPS.

You need to test the MAT and CTS for proper resistance at 3 temperatures.

One major, very common possibility is a poor TPS ground that is changing resistance and leading to a variable idle , loose, dirty ground. Test the TPS ground wire on the ECU side!!! Should be less than 1 ohm to the battery negative post. Also test the TPS idle voltage with the elevated idle to see if the TPS has gotten sloppy (dirty, noisy, inconsistent output) internally.

Clean all the battery and vehicle main ground connections. You want bare metal on both faces of the connection.

I had to run to new TPS sensor ground wires directly to the battery on my 87.
Click to expand...
Mike's on to something here.

I was having the same problem on my wife's Jeep off and on for a few weeks. I hooked up the factory tester and it stopped me with a code C1004 sensor ground issue. I unplugged the TPS, squeezed the female split connectors slightly, added dielectric grease, and it's been good since. TPS ground circuit has been an issue over the years.

Have you ever performed a complete ground refreshing on your Jeep? It takes about 1/2 hour and can keep gremlins from appearing in the future.
 
Thanks for the input guys - I have recently cleaned and tightened grounds. I haven't tested the TPS ground from ECU, however.


I did take out the IAC, cleaned it and the hole it goes into very well. They are both essential spotless at this point and the trouble continues, much the same as it had before. So unless that bugger is toasted, I don't think it is the problem.

On a whim (and not knowing what I should expect to see) I backprobed the TPS with my multimeter while the jeep was running (figure it should show me the same as when testing with it just powered on but not running?) -- and the Voltage output readings are all over the place


At this point I'm thinking either TPS -- or the wiring to the ECU? I haven't ever touched that portion of wiring or the ECU at all; if there is any advice or instruction that will help me please don't hesitate to shoot it my way.

Thanks!
 
Ecomike said:
You need to test the MAT and CTS for proper resistance at 3 temperatures.
Click to expand...

Sorry! Forgot to address that in my last reply:
I haven't tested them at 3 temperatures, but I did check them yesterday while working on her. I used an IR therm on the Manifold next to the MAT to estimate the temp -- it was at about 85F (that sound close?) and the R was right around 2600 Ohms which looked right for the chart. I checked the CTS after that --- gage was showing just over 100 F and that showed right at 1600 ohms. I'll check them both before any starting today - that should give me another reading to go on.
 
With power off, use the ohms function on DMM meter and test between the ground wire at the wiring harness TPS connector (while not connected to the TPS) to the battery negative post. Any thing over 1 ohm is too high!!! Also test while wiggling wiring harness assemblies to see if it jumps! I spend nearly 2 years fixing everything thing else under the hood on my 87 when I bought it, before I found an 8 ohm average, but variable ground on the ECU to TPS sensor wire. 8 Ohms does not sound like a lot but if it changes from 4 to 8 ohms for instance, it changes the idle by about 600 rpm!

When I found the bad sensor grounds in the harness, I just spliced in new ground wires and ran them straight the battery negative post.

Also check the input voltage to the TPS while running from the input wire (5V from the ECU) to the battery negative post to see if the 5V from the ECU is steady or bouncing around!!!!!
 
Last edited:
With power on, resistance from Ground pin on TSP connector to Negative terminal is a fairly constant 202 ohms - so now I know what is most likely causing the issue, just have to find the source.

Thank you again for the input. If anyone has information on the route that ground takes, I'm happy to receive this for tracing as well. Does the ground go to ECU and then out, or from connector to engine block? I'll start checking the FSM to see if I can find where that is, but I'm not intimately familiar with it yet.
 
Well, it was definitely a ground issue -- traced my way all the way to the dipstick connection...so I cleaned it all back up and viola - starts and idles perfectly. Meh. I'm not sure what I can do to prevent the loss of connectivity there? I'd used di-electric grease previously, maybe I didn't make it **tight** enough after that. It is currently as tight as I can get. Time will tell if it starts going south I'll see what else I can do.
 
box_goblin said:
Well, it was definitely a ground issue -- traced my way all the way to the dipstick connection...so I cleaned it all back up and viola - starts and idles perfectly. Meh. I'm not sure what I can do to prevent the loss of connectivity there? I'd used di-electric grease previously, maybe I didn't make it **tight** enough after that. It is currently as tight as I can get. Time will tell if it starts going south I'll see what else I can do.
Click to expand...


Whooda thunk it? People act like cleaning the ground connections at the dipstick tube stud is some kind of voodoo and should be disregarded. Congratulatoins on your success and thanks for listening to us old farts who have been there and done that. Me and Mike have the t-shirts, too.
 
It's always good to stay well grounded in life!:cheers:
 
when i start mine it immediately goes to ~2500-3000 then after a few seconds back down and it only goes that high when starting, idles just fine well actually kinda low around ~250-500 but i know im mis-firing so that could be a cause, any suggestions?
 
I would check for a leaky fuel injector, and wait, hmm, let me think........ check the grounds maybe?????

Also, the B+ latch relay may not be resetting the IAC when you turn off the jeep. That will cause a high starting idle too. So search the procedure for that. It is one of 4 relays in a neat little row on the passenger side under the hood.
 
You cannot overground Big Alpha.

I always suggest refreshing the grounds but I also highly recommend adding more. One of my favorites is adding a 4 gauge cable from the firewall, where the POS braided strap is, over to a bolt on the intake manifold. Another one I like to do is a cable, #4 is good here too, from the dipstick tube stud to an 8mm stud on the passenger side inner fender just below where the shock mounts. NO matter where yo uadd a ground, always make sure the existing connectors are clean and shiny and there is no paint on the mounting areas.
 
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