jesses89cherokee

I have an 89 cherokee. cant find anybody wih a interface for the connection to connect to an odb 1 scanner. it usually happens when engine is warm. but it will hesitate from an idle to a roll. as soon as i hit the gas it sputters. and i either have to let off and try again or it dies. it also happens idling while rolling. press the gas and it has the hesitation for a second then continues to operate fine. engine will sometimes start, rev high (3k rpms) then idle correctly. i have eliminated the possibility of it being an egr valve. distributor, plugs, wires, crappy gas or dirty injectors, fuel pump is new or so i was told when i bought it. any other input or ideas would be awesome. thanks in advance

7dark

89 jeep cherokee is a renix so an obd 1 tool will not fit it. As for the problem, maybe a partially clogged fuel filter? If you haven't changed it in awhile its probably a good idea to anyways. I'm not sure if im allowed to post links so ill pm you a link that tells how to check the filter.

jesses89cherokee

okay. changed the fuel filter n that didnt work... could it possibly be the tps?

5-90

The only pre-1991 XJs that were OBD-complaint were the 1984-1986 w/V6-173 - since AMC bought that engine with GM's control system.

Easy test - take an analogue ("sweep needle") ohmmeter and check the function of your TPS. You don't need the keys to do this - you'll be disconnecting the sensor anyhow. I can never recall the terminals you want to connect to, but pull the three-pole connector (if you have two) and connect the ohmmeter to the sensor, then run the throttle blade by hand. If you don't see the needle on the meter move, even on the lowest setting, switch to a different set of two terminals.

What you're looking for isn't an absolute value, but smooth and even change in resistance as you move the throttle valve. Chances are good, you'll find a flat spot (I went nuts figuring this out the first time I had to deal with it, and I've fixed it for four or five more people since.)

What happens is that the TPS is a carbon-track potentiometer - which means it's a variable resistance based on a carbon (graphite) curved track using a metal wiper arm and button to make the sliding contact. When you spend a lot of time at part-throttle, the graphite will gradually get compressed (slowly!) and the resistance changes in that spot. It will typically manifest as a "flat spot" or a "drop-out" in the response curve.

You must use an analogue ohmmeter, because a digital won't respond quickly enough to show it. Digitals have buffer circuits inbuilt to stabilise the display, or you'd never be able to read the thing.

The engine hitch happens because the ECU looks at the voltage output from the TPS (along with engine vacuum and coolant temperature) to determine engine loading. Engine loading data is combined with intake airflow data (MAP and IAT, primarily) to compute fuel metering - and both the loading and airflow data are needed! When the loading data gets fluky, so does input fuel metering.

I've noted this little irritant, typically, when the sensor is between 16 and 22 years old - and it will usually happen around cruise RPM.

jesses89cherokee

wow dude you really know your ****. i will find an analogue ohmeter. what should its reading be at an idle? but just run it up and down as if i were using the throttle and it will jump or skip in resistance? im somewhat mechanically inclined but not to your expertise. i really hope thisis it. ive thrown so much money at this thing cause i cant put it on a freakin odb. sux but what can you do. thanks alot. i will keep you posted on my discoveries. thanks again!

5-90

You won't be testing the sensor under power - you'll be disconnecting it. Engine OFF.

The absolute value of the resistance is unimportant - what you're watching is the change in resistance as the wiper arm moves.

The throttle position sensor is a centre-tapped variable resistance that is used as a voltage divider on a reference voltage provided by the ECU - this is why it's important that a new TPS is properly adjusted with an output voltage that is 17% of actual input voltage. The percentage of the reference voltage making it back to the ECU is what tells it how far the throttle valve has been opened. This is why the actual resistance isn't critical - just the change in resistance as the wiper moves.

As far as OBD, I'm not wholly impressed with it. I cut my teeth on early Corvettes and Camaros (restoring them with my uncle, starting when I was about seven years old...) and I'm too used to doing my own thinking. Besides, I've had plenty of cases where the OnBoard Diagnostic system was just plain wrong. It's far less common with OBD-II than with OBD-I, but it still happens.

I've taken mechanic's training courses (wanted to see what I'd missed in the last thirty years of swinging wrenches.) They're trained to be "technicians," not "mechanics" - and most of them can't think around OBD/OBD-II and get all screwed up if the system is wrong.

Most of the RENIX sensors can be tested with a DMM and a little know-how, you've just happened on one of the very few (two, I think) tests that will require an analogue meter. You can find the RENIX FI manual at Pirates of the Rubicon (page 58 duplicated, no page 59. I'm workin' on it...)

jesses89cherokee

okay so i took it to a shop today. they did the test to check for a flat spot and found nothing. i am really at whitts end with this thing. i pulled out all spark plugs n everything is good. no carbon build up. does anyone have any more insight on what the hell is wrong with my jeep? anything would help. thank you

greenmachine91

it could be your mass air-flow sensor dude, go ahead and check that if yours has one. it should be on the throttle. does it hit 3k at cold start or even if warm?

jesses89cherokee

i think it has a map sensor. manifold absolute pressure or something like that. anyone know the troubleshooting voltages or resistances of the MAP sensor?

jesses89cherokee

it came down to the coolant temp. sensor! wow. never thought that would have affected it making it fuel like a fuel problem. thanks for all the help!