Trying To Eliminate/narrow Down Possible Idle Issue Causes

[revcor]

Let me start off by saying I know there is a surging idle checklist somewhere, and I'm not trying to avoid using it out of laziness. I'm just hoping that before I start with that, I can eliminate some possible items or get pointed in the direction of some that seem more likely.

I have an '89 LX (stock high mileage motor except for Vortech V3 SCi kit, and full catless exhaust) that has a very intermittent idle issue. 90% of the time it's totally fine, idling around 700 or so. But every once in a while (could be as often as a few times a week, or sometimes goes months without any issues at all) one of a few weird things happens, not seeming to correspond with any certain conditions:
a) At a stoplight or elsewhere idling in neutral, idle will rise up to 1200-1400 and just hang there, lasting anywhere from a few minutes to the entire rest of my drive (up to 20-30min is the longest I've noticed). During this time if I'm driving and push in the clutch or shift into neutral, revs will drop very slowly to the 1200-1400 point
b) A less extreme, and more surgey version of (a).
c) A handful of times, starting the car up cold, as soon as the motor turns over it jumps up to 2000rpm and stays for a good 30-40 seconds (much more aggressive than the occasional slightly high idle to warm up) (quite alarming at 8am)

A few times when it's idling high, I've noticed the motor will have a tiny "cough" or "stumble" and a very brief drop in rpms (I always imagine somebody running too fast while they're half asleep and accidentally catching their shoulder on a doorway).

Other things that may or may not have any connection with this issue:
a) intermittent check engine light (flashes quickly or lasts for a few seconds). Maybe a quick flash every 5-10min, sometimes more frequently. (Also doesn't seem to be related to any particular conditions).
b) ran self-test, and has codes 31 and 53 in CM. I've seen 31 refer to an EGR issue and also an EVP issue, not sure which in my case. 53 I believe is TPS voltage too high. Not sure if the intermittent flashes of the check engine light are due to one, or both, of these.

I've checked the TPS voltage and it was set where recommended (around .9-1.0v, it was a few months ago when I did this). I cleaned the salt and pepper connectors. I cleaned the IAC, thinking maybe it was spazzing out. I'm hoping this peculiar combination of issues and codes might help people point me in a particular direction.. when it idles high it gives me anxiety.. Thanks in advance I know it's a lot to read.

 

[Noobz347]

What size injector?
What throttle body?
How much boost?
Have you pulled codes or do you get a check engine light when the stumble occurs?

 

[Dan02gt]

I've seen a TPS cause that before and being that you got a TPS related code I think I would start there.

 

[revcor]

What size injector?
What throttle body?
How much boost?
Have you pulled codes or do you get a check engine light when the stumble occurs?

Stock injectors and throttle body, and 5-6psi. It's the vortech kit designed for stock motors. I pulled the codes and got 31 and 53, and the check engine light flashing is not related to the stumble, it'll just come on randomly while cruising.

I've seen a TPS cause that before and being that you got a TPS related code I think I would start there.

If I've checked the voltage and its within spec, what would you suggest next to deal with it?

 

[Steel1]

I've also had a tps cause that issue, but got code 63 (tps below min. voltage).
Checked tps voltage it was fine, checked for 5v ref signal also fine.
Installed a new tps , issue and code went away.

 

[Dan02gt]

A TPS can have a completely dead or intermittently dead spot in its sweep range. So it can test good at idle and WOT and be completely off at 50% throttle. I don't really like to throw parts at problems, but as cheap as a TPS is I think I would just swap it with a new one.

 

[Mustang5L5]

While the TPS is in spec at idle, it could be an intermittent internal electrical issue causing it to read high. It does make sense as when the TPS advances, the timing does as well and your idle will rise slightly.

There's enough evidence here that I would try and replace the TPS with a motorcraft unit. If you really want to confirm it, reset all the codes, and drive around a bit more until you see that CEL flash on at least once. Then pull the codes and see if the 53 came back.

 

[Noobz347]

Stock injectors and throttle body, and 5-6psi. It's the vortech kit designed for stock motors. I pulled the codes and got 31 and 53, and the check engine light flashing is not related to the stumble, it'll just come on randomly while cruising.


If I've checked the voltage and its within spec, what would you suggest next to deal with it?

Two things come to mind outside of the Surging Idle Checklist:

You must be running an FMU. It may be time to change or rebuild it.
The other thought that comes to mind is the possibility of a leaky injector.
An injector that is sticking do to the elevated pressure changes resulting from the FMU.

Think about those things as you recall the sequence of event during these idle stumbles.

 

[jrichker]

Code 53 Throttle Position sensor too high – TPS – TPS out of adjustment, bad connections, missing signal ground, bad sensor.

Wire colors & functions:
Orange/white = 5 volt VREF from the computer
Dark Green/lt green = TPS output to computer
Black/white = Signal ground from computer This is a must have ground -no options, it has to work correctly for the TPS to work. If this ground is intermittent, it is likely to be the source if your problem.

Always use the Dark green/lt green & Black/white wires to set the TPS base voltage.

Do the test with the ignition switch in the Run position without the engine running.

Use the Orange/white & Black white wires to verify the TPS has the correct 5 volts source from the computer.

Setting the TPS: you'll need a good Digital Voltmeter (DVM) to do the job. Set the TPS voltage at .5- 1.1 range. Because of the variables involved with the tolerances of both computer and DVM, I would shoot for somewhere between .6 and 1.0 volts. Unless you have a Fluke or other high grade DVM, the second digit past the decimal point on cheap DVM’s is probably fantasy. Since the computer zeros out the TPS voltage every time it powers up, playing with the settings isn't an effective aid to performance or drivability. The main purpose of checking the TPS is to make sure it isn't way out of range and causing problems.

The Orange/White wire is the VREF 5 volts from the computer. You use the Dark Green/Lt green wire (TPS signal) and the Black/White wire (TPS ground) to set the TPS. Use a pair of safety pins to probe the TPS connector from the rear of the connector. You may find it a little difficult to make a good connection, but keep trying. Put the safety pins in the Dark Green/Lt green wire and Black/White wire. Make sure the ignition switch is in the Run position but the engine isn't running.

Here’s a TPS tip I got from NoGo50

When you installed the sensor make sure you place it on the peg right and then tighten it down properly. Loosen the back screw a tiny bit so the sensor can pivot and loosen the front screw enough so you can move it just a little in very small increments. I wouldn’t try to adjust it using marks.

(copied from MustangMax, Glendale AZ)

A.) Always adjust the TPS and Idle with the engine at operating temp. Dive it around for a bit if you can and get it nice and warm.

B.) When you probe the leads of the TPS, do not use an engine ground, put the ground probe into the lead of the TPS. You should be connecting both meter probes to the TPS and not one to the TPS and the other to ground.

C.) Always reset the computer whenever you adjust the TPS or clean/change any sensors. I just pull the battery lead for 10 minutes.

D.) The key is to adjust the TPS voltage and reset the computer whenever the idle screw is changed.

The TPS is a variable resistor, must like the volume control knob on a cheap radio. We have all heard them crackle and pop when the volume is adjusted. The TPS sensor has the same problem: wear on the resistor element makes places that create electrical noise. This electrical noise confuses the computer, because it expects to see a smooth increase or decrease as the throttle is opened or closed.

TPS testing: most of the time a failed TPS will set code 23 or 63, but not always. Use either an analog meter or a DVM with an analog bar graph and connect the leads as instructed above. Turn the ignition switch to the Run position, but do not start the engine. Note the voltage with the throttle closed. Slowly open the throttle and watch the voltage increase smoothly, slowly close the throttle and watch the voltage decrease smoothly. If the voltage jumps around and isn’t smooth, the TPS has some worn places in the resistor element. When the throttle is closed, make sure that the voltage is the same as what it was when you started. If it varies more than 10%, the TPS is suspect of being worn in the idle range of its travel.

Adjusting the TPS fails to resolve the problem:
Check the black/white wire resistance. Connect one ohmmeter lead to the black/white wire on the TPS and one lead to the negative post on the battery. You should see less than 1.5 ohm, more than that indicates a problem. Always take resistance measurements with the circuit powered off. Do the wire jiggle test while making the measurements.

Clean the 10 pin salt & pepper shaker connectors.

.[/b]

 

[revcor]

While the TPS is in spec at idle, it could be an intermittent internal electrical issue causing it to read high. It does make sense as when the TPS advances, the timing does as well and your idle will rise slightly.

There's enough evidence here that I would try and replace the TPS with a motorcraft unit. If you really want to confirm it, reset all the codes, and drive around a bit more until you see that CEL flash on at least once. Then pull the codes and see if the 53 came back.

I tried the two local Ford dealerships and they both confirmed that you can't get a TPS from Ford anymore, so I'll have to check summit or rockauto and see if they have Motorcraft ones.

Two things come to mind outside of the Surging Idle Checklist:

You must be running an FMU. It may be time to change or rebuild it.
The other thought that comes to mind is the possibility of a leaky injector.
An injector that is sticking do to the elevated pressure changes resulting from the FMU.

Think about those things as you recall the sequence of event during these idle stumbles.

The kit does have an FMU, but it's only been on the car for a year and a half or so, seems odd that it would require a rebuild or replacement so soon. How would I diagnose a leaky injector? And unfortunately there is no rhyme or reason to the weird idle behavior; I haven't been able to notice any patterns or common denominators.

 

[Mustang5L5]

Rockauto.com is usually good for motorcraft parts.

Part number is CX1133, and I see rockauto has them for $38. Amazon has them for $35.

 

[jrichker]

@revcor
Someone didn't read my code 53 post. You need to check the wiring before you replace the sensor. Do the wiggle the wire test when you check the wiring resistance!!!

Clean and check the TPS signal ground connections and 10 Pin connector that has the signal ground - white connector, pin #1

 

[revcor]

@revcor
Someone didn't read my code 53 post. You need to check the wiring before you replace the sensor. Do the wiggle the wire test when you check the wiring resistance!!!

Clean and check the TPS signal ground connections and 10 Pin connector that has the signal ground - white connector, pin #1

I did i promise! But I'd done most of those steps already (at least checking the voltage and cleaning the salt and pepper shakers and the TPS connector) so I did admittedly skim it (my mistake). I missed the checking the resistance part and the wiggle test part.. How do I perform the wiggle test?

 

[jrichker]

A wiggle test is just that: you wiggle the wire and connections while measuring the resistance. Disconnect the white 10 pin connector and connect 1 test lead from the meter to the center pin of the engine mounted harness. Use a safety pin to probe the black wire on the back of the EGR sensor connector and connect the meter test lead to it. Set the meter to either continuity or low Ohms and wiggle the wire and connector while monitoring the meter.

 

[revcor]

Update: I borrowed an analog multimeter from my dad, and did the throttle range test, and it seems fine, the needle moved smoothly throughout the range between ~.9v and ~4.5v. However, I did the resistance test from the black wire coming out of the TPS to the negative terminal of the battery and instead of the less-than-1.5 I was supposed to get, it was slowly fluctuating from 36 up to 50 and then back down again. Then I disconnected and reconnected the probes, and now it shows a steady 14. Maybe I had a bad connection. When I wiggle the black wire the reading fluctuates +/- 0.1. I then tested resistance from TPS black wire to the center pin of the engine-mounted part of the white 10-pin connector and got a reading that fluctuates slowly between 17-18. Jiggling the TPS wire again didn't do much, but wiggling the 10 pin connector makes the reading jump up to 20-25 until I stop moving it.

Hopefully this is thorough enough that you guys will be able to cull some clues from my description.. I wonder of my readings ate off by a power of 10, since you said to look for less than 1.5 ohms and I got 14, or if they're genuinely that much higher than they're supposed to be.

 

[revcor]

This is the multimeter and setting I used for checking the resistance if that helps

 

[jrichker]

What reading do you get when you short the meter probe tips together on the lowest Ω scale?

 

[revcor]

What reading do you get when you short the meter probe tips together on the lowest Ω scale?

Just tried, it hovers between 0.4 - 0.6

 

[jrichker]

Just tried, it hovers between 0.4 - 0.6

That sounds right. with that in mind, You should see 1.0 -.1.5 Ohms when you test the TPS connections; 14 Ohms is out of tolerance.

 

[revcor]

That sounds right. with that in mind, You should see 1.0 -.1.5 Ohms when you test the TPS connections; 14 Ohms is out of tolerance.

So with my results of 14 ohms TPS-to-battery, 17 ohms TPS-to-10-pin connector, and 22 at the 10-pin when jiggled, what would be your next investigative step? Also how could the resistance be higher at the 10-pin than at the battery? Wouldn't the resistance over a longer circuit have to be equal or higher?