88 4.0 No Start...faulty TPS?

So last week I did a good amount of work on the heep (borla header, poly motormounts, new cap, new rotor, new plugs, new vac lines, new CCV lines, oil/filter change, and a k&n "cold air"). Got the truck running fine with all these new additions (the best it's been running in a while in fact). Went on a short trail (about 4.5 miles total) and the truck performed amazingly and eventually drove it home alright.

The next day I started it up and it idled fine. As soon as I stepped on the gas the engine began to run really rough and eventually died. It then wouldn't start after that. I checked all the vacuum lines and electrical connections and everything was ok. I wiggled the injectors and tried starting it again, and it did. This time it idled fine for a minute and then ran rough and died (no throttle applied). :moon:

I just tested the TPS today following both a haynes and chilton manual. If I performed the test correctly, then according to those books the TPS is bad. I got a reading of around 4.8 volts which was constant from closed throttle to wide open (and everything in between). I'll go out and double check it tonight. So say the TPS is bad...would that cause the no start? If it is bad, is it a dealership part or could I get away with one from pick n pull..? :wierd:

Also..I remeber partially while I was swapping out the old header and putting in the borla, I briefly cleaned the o2 sensor with carb cleaner. Would this destroy it? If so, would it cause the problems i'm describing? :hang:

I'm a pretty competent home mechanic, but this has throughly stumped me. I'm searching this site and my haynes/chilton manuals like a mad man trying to get my damn truck running!! Any help is, as always, greatly appreciated :thumbup:

i do believe you fried the o2 sensor but with the tps it says you idleing so it shouldent stall till you give it gas. but the 4.8 v might think you full throttle. which puts it in a flood clearing mode. i think not positive

90xj06 said:

i do believe you fried the o2 sensor but with the tps it says you idleing so it shouldent stall till you give it gas. but the 4.8 v might think you full throttle. which puts it in a flood clearing mode. i think not positive

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Thanks for the quick response 90xj! I tested the TPS again, only this time alittle more precisely and sure enough I believe it to be toast. It read 4.88 v constantly.

Again, is it possible to get one of these cheap? It looks like i'll be needing a new TPS and o2 sensor :rattle:

Jeez from both kragens and napa, a new TPS is around 90-100 and a new o2 is ~55 :rattle: Is there another way to check the TPS before I go ahead and buy a new one?

just make sure that you have the right wires that your testing. then unplug the sensor and use an ohm meeter and see if it changes.

but now that i think of it it is most likely the tps sensor. if you do go to a from a junk yard just make sure it works before you leave.

NEVER! get a tps sensor wet!

try unplugging it and see what happens. it might not start.

I have never in 10 years of working on XJ's found a bad TPS that 4.8 volts constant 99.9% of the time it will be dead spots or nothing. Your reading the wrong wire.

Humph well which two wires is it? Chilton says blue and gray (if i'm remeber correctly offhand).

Two wires? there should be three for the engine TPS.

Well yea, but the ones I test.. (Chilton says you test 2 wires)

You want to measure BLK - to BLU + GRY is 5 vdc in.

Oh..hahahahahah no wonder.


:anon: :eyes: :roflmao:

I honestly thought that I was doing something wrong...

Take all mesurments from BLK.
BLU will be .87 to 4.8 and GRY will be 4.8

for some reason i had .25 volts on my negative terminal on my battery to the tps ground. so i ran a make shift wire and now its better

Well, sorry for the confusion, but the TPS appears to be in working order. I adjusted it today to read ~ .86 volts at idle and ~ 4.70 at wide open.

Could it be my ignition coil? Or fuel delivery related problem?

Well have you tested the CPS, do you have spark?

The way I tested to see if I had spark was I unplugged one of the plug wires and hooked up an old spark plug to that. Cranked the engine and no spark. then I unplugged the wire from the ignition coil to the cap. Again plugged the old spark plug into that. Cranked the engine and no dice. Then I put the plug close to the engine block (a good ground) and cranked, again no spark.

Most likely i'm doing it wrong, please enlighten me :thumbup:

I unplugged the CPS and tested for resistance in it. The meter kept flickering, as if it had resistance (normally isn't the CPS by itself an open circuit?). After acouple seconds of this it would stop and display no resistance, meaning an open circuit (so, is the CPS good?) A visual inspection of the wire where it snakes between the intake/head shows that it's been melted slightly by the header. There is no section that is burnt completly through (I'm assuming that the wiring could potentially be shorted).

Most likely i'm doing it wrong, please enlighten me :thumbup: :thumbup:

I had an extra fuel filter, so I replaced that too. Still it wont start... :smsoap:

A cps hot should ohm out between 200-300, IIRC. An open circuit really makes me think it's bad. and a ad CPS would cause no start.
Sounds like you're testing for spark properly. It's a likely case that the CPS shorted to the header and now it's toasty.
Try unplugging and replugging the CPS. Sometimes that coaxes it back to life for a bit. If taht does it, then off to the store for a new one.
Good thing is that Renix CPS are cheap.
~$30

Awesome, thanks for the advice. I'll get to it after work. I'm not able to actually get the truck running, so I can't test the 'hot' cps. (hence why i was just testing the circuit to see if it was open or not)

It's a biatch to replace isn't it? Locating it took me like 20 minutes :wierd: Good thing it's cheap!

Replaced the o2 and it runs fine.. I'll be taking it for a real test drive later. Also i've got a new CPS on the way. :thumbup:

Thanks for the help fellas! :farmer:

great tho here your all better.