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Old 05-09-2014, 08:16 PM   #1
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1989 2.5L Throttle Body Issues

I have a 1989 jeep yj with the 2.5l with throttle body injection and I am having some major issues. First off when you first try to crank it it will start and then die then you have to hold the pedal to the floor to get it to start(like it is flooding out). The second problem is a little more major. Once you get it started it idles fine and runs fine as long as you slowly give it gas, but if you go from idle to wide open throttle it backfires through the throttle body and then catches up. I have rebuilt the throttle body, put a fuel filter on it, replaced the fuel pump, and replaced the spark plugs. It also smells like it is running super rich. To me it seems like the timing is off but there is no way to adjust it. What could the problem be? Any info is greatly appreciated. Thanks

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Old 05-10-2014, 12:08 AM   #2
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Coolant temp sensor on intake manifold was my problem and I had the exact same problems. There are two sensors for coolant temp one is on back of cylinder head for the guage. It is on back right side looking from front of jeep. The one u want is on the intake manifold itself and is the one going for the computer. Mine was so bad my oil was full of gas also. The computer uses this signal to know when the engine is warm and to adjust fuel and timing so that your engine runs smooth and economical.

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Old 05-10-2014, 06:07 AM   #3
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Did it do all this BEFORE you rebuilt it?
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Old 05-10-2014, 08:58 AM   #4
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I will try changing the sensor. Yes it had the issues before I rebuilt the throttle body. That's why I rebuilt it. Could it also be an oxygen sensor and is there any way to test the coolant sensor?
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Old 05-10-2014, 10:05 AM   #5
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I tested the coolant sensor. The resistance decreases as the jeep warms up and increases as it cools down. Does this sound right?
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Old 05-10-2014, 10:39 AM   #6
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Coolant sensor is acting properly.

Lets go back to the throttle body and its rebuild reassembly

Mine after rebuild had a serious over fuel problem and ran like crap. When I disassembled I found an issue

When you reassembled the pressure regulator you have to line up the gasket for the over full return of fuel. Its three bolts and a gasket it has a hole that corresponds to over pressure fuel return baclk to the tank. I had mine wrong and it was loading up and failed smog.

Take 30 minutes remove and check it could be as simple as that.

Don't chase anymore sensors until that has been confirmed its cheap and I always get a beer after.

Let me know what you find.
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Old 05-10-2014, 10:49 AM   #7
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It could be the 02 sensor yes.

Have you checked for codes?

Heres a link to how to do that on OBDI Jeeps if you don't know how:



Remember, there can be codes with no light, but on mine the O2 sensor lit up the CEL
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Old 05-10-2014, 12:21 PM   #8
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I checked the gasket in the throttle body and it was good. Is there supposed to be an adjustment on the bottom of the little bowl thing? On the inside of mine there is threads that look like they adjust, but on the outside it looks like someone has filled the hole where the threads go into with lead. I also checked the fuel pressure and it is running about 18psi. I think I read somewhere that it should be about 15 psi. Should I be concerned about that? I also tried to check the codes but I can't even get the check engine light to come on. Maybe a computer problem?????
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Old 05-10-2014, 02:09 PM   #9
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The TBI system should be running about 14-15 psi at idle. If it isn't, you may need to adjust the fuel pressure regulator. Turn the screw out (counterclockwise) to decrease the pressure, turn it in (clockwise) to increase pressure. See if you can adjust the pressure to within spec. (Note: sometimes these adjustment screws are plugged at the factory so that the owner doesn't mess with them and adjust them out of spec. You can try and carefully drill out the plug, but be careful not to damage the screw head).



If you can't adjust the fuel pressure to within spec, the fuel pressure regulator may be bad or there may be a blockage/restriction on the return line to the tank.

Regarding the codes - Codes 12, 33 and 55 are normal and should appear every time you read the codes. Turn the key to "run" three times - ON-OFF-ON-OFF-ON. If you can't even get these codes to appear doing this, you need to find out why. There could be a problem with the wiring or with the computer itself. I am not knowledgeable about the computer wiring on your Jeep, and the Haynes manual I have isn't of much help with wiring, so I can't be of much help with specific diagnosis there. But maybe someone who knows the 2.5L TBI setup will chime in.
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Old 05-10-2014, 03:32 PM   #10
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I got to the adjustment screw but I didn't have any luck turning it. When I checked the fuel pressure before I had it inline but I just checked it through the test port on the throttle body and got 16psi. I also checked the oxygen sensor and it isn't doing anything.
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Old 05-10-2014, 05:32 PM   #11
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The adjustment screw is soldered in place. Get a cheap plumbers torch heat it up and remove it clean the threads and install adjust as required.

It's soldered believe me I know.

Good luck
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Old 05-10-2014, 06:49 PM   #12
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I got the solder out but I can't turn the screw. I've thought about drilling it out and taping a new hole but I'm not sure about the little ball thing on top of the screw inside the little bowl.
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Old 05-10-2014, 07:14 PM   #13
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Heat it up again and try to move the screw it will come out.
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Old 05-10-2014, 08:36 PM   #14
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Do you think that will fix my problem even if its just 1psi over?
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Old 05-10-2014, 08:57 PM   #15
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There is a spec for a reason so me YES it would make a difference. While you working an issue I would make all in spec.
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Old 05-10-2014, 09:32 PM   #16
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I moved the screw but nothing happens. Still cuts out an runs rich. While I had the throttle body off I noticed there is gas sitting in the intake.
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Old 05-10-2014, 09:39 PM   #17
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Because you didn't specifically mention it I have to ask: Did you adjust the fuel pressure to within spec and still have these symptoms?
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Old 05-10-2014, 09:42 PM   #18
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No I didn't put a guage on it but it is still running way too rich even if the screw is turned out as far as it can go. This tells me that it doesn't do anything. I will put the guage on it tomorrow to see but I don't think it is changing things one bit.
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Old 05-11-2014, 04:10 PM   #19
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The only thing adjusting the screw without checking the pressure at the same time tells you is that you still have the same symptoms. Until you verify that the pressure is within spec you still have potential causes that need to be ruled out.

1.) Until you check the pressure while adjusting it, you don't know if the pressure regulator actually adjusts the pressure. If it doesn't adjust pressure at all (increase or decrease), then you have a problem with the regulator. If it adjusts pressure, but can't drop the pressure enough to get it in spec, it could be a problem with in the fuel return line (something causing too much back pressure due to a restriction).

2.) If pressure is adjusted within spec and you still have the symptoms, then I'd check for leaky injectors. To do this, unplug the injectors and have someone briefly crank the engine while you watch the injectors. Good injectors will only spray when they get the signal from the computer (unplugging the connectors stops them from getting this signal). If you get fuel spray or drip from the injectors while doing this test, then the injectors are bad.

3.) If pressure is fine, injectors don't leak, but you still have the symptoms: there could be a problem with the computer sending bad/wrong signals. Ideally, you'd check engine codes first before tinkering with sensors or doing anything else. Without any codes, you'll have to guess if it is a bad sensor or if the computer itself is bad. You can replace sensors at random, but codes are the most cost effective way to find a bad sensor.

If you can't get the computer to even display the default codes, you have another problem to fix. The lack of displayed codes might just be a burned out bulb, or could be bad wiring or even a bad computer. Yet another unknown. If you reach the point of replacing all the sensors and still no fix, the problem may lie with the computer.
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Old 05-11-2014, 06:41 PM   #20
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There is some really great info in this thread by all who contributed and here is my take after doing some research and my own experience with an 89 2.5L that couldn't pass smog after an overhaul. This is not a cheap shot at any info provided previously and is only my opinion.

I have read everything in thread over again and spent some time in my FSMs.

Here is what I find

89 2.5L will not produce codes with a key only 91 and newer because they don't have a Check Engine Light system only the standard emissions timer which is not a player here.
Self-Diagnostic Codes: Retrieval and Mysterious Meaning. - JeepForum.com

There is only one injector in a throttle body 2.5L from 89 and you can observe it while it runs down the throttle body opening. You can't tell if it is leaking replacements are about $45 at O'Reilly with a core charge.

The adjustment on your fuel pressure regulator was fine before this happened and was soldered in place at the factory so no one has messed with it.

OP stated "Once you get it started it idles fine and runs fine as long as you slowly give it gas, but if you go from idle to wide open throttle it backfires through the throttle body and then catches up. I have rebuilt the throttle body, put a fuel filter on it, replaced the fuel pump, and replaced the spark plugs. It also smells like it is running super rich. To me it seems like the timing is off but there is no way to adjust it. What could the problem be?

Stop everything. Backfires on acceleration is the key so lets look at some basic troubleshooting right there.

Read the plugs I will attach the chart.

Remove them and see what they say

While the plugs are out do a compression test hot or cold you may have a compression issue with a valve.

Once that is complete and you know the internals are not involved then proceed to find out about the timing.

Do you have a timing light??? Have you checked the timing at idle should be close to 8 degrees BTDC.

Check your harmonic balancer and make sure it looks pristine and hasn't slipped if you get an real oddball reading.

Once you determine the timing is good at idle advance the throttle and see if the timing advances.

Have you removed or installed the distributor during all of this???

Has the timing chain ever been replaced???

Need some answers to get any further but I am still reading.

Take no offense to this post. Its just me trying to help out.
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Old 05-11-2014, 07:44 PM   #21
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The compression on all cylinders was 130.
The plugs were dark black, so oil fouled.
I haven't checked the timing because I saw no reason to check it if I couldn't adjust it.
The distributor has not been removed.
I am not sure if the timing chain has been replaced.
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Old 05-12-2014, 10:41 AM   #22
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All good news.

Now if it continues to run rich after reaching operating temp one if the sensors that inputs to ECU is not working and it gets five inputs.

O2 sensor
TPS
Intake Air Temp
MAP

I am at work I'll finish tonight
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Old 05-12-2014, 12:20 PM   #23
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Thanks Jokerchief462 for pointing out the lack of a check engine light in the 89. With my limited knowledge of the 2.5L I forgot that the older fuel injected Jeeps used the Renix system. DOH!


WILL BELL- After tinkering with the regulator, did you ever get the fuel pressure adjusted back to within spec?
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Old 05-12-2014, 05:23 PM   #24
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Yes the fuel pressure is in spec. I changed o2 sensor, cap, rotor button, plugs, and wires yesterday. Where is the intake air temp sensor located?
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Old 05-12-2014, 08:24 PM   #25
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Here is the intake air temp sensor and the temp sensor charts.

I removed both of mine chasing problems and did the ohms checks from the freezer to the oven takes awhile but mine were actually good.
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Old 05-13-2014, 10:51 PM   #26
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Hey guys I just read through this and I had a very similar issue happen with my jeep when I got it, and it turned out that my injector was missing the lower O ring and that was causing the intake to fill with fuel and the motor to run slightly but eventually die from flooding, and if I tried hard enough it actually hydro locked on fuel in the cylinders and caused the rotor to advance timing by skipping a few splines. But if you nicked the lower ring when you put the injector back in it could be doing the same thing but just not as drastic. Just a thought because I didn't see that you had checked it
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Old 05-14-2014, 06:32 PM   #27
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Thanks I will check that. I also have a throttle position sensor on the way.
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Old 05-15-2014, 08:26 PM   #28
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Changed the throttle position sensor and the map sensor, but NO CHANGE. It runs the best when the vacuum line to the map sensor is unplugged and capped off?????? I am to the point now to were I am ready to start asking how to swap a 350 into it. This is really making me mad because I can't stand something that doesn't run right. I've tried multiple times to blow it up but with no success due to my desire to get it running right. I feel like it is still getting way too much fuel. Anybody have any other suggestions????
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Old 05-16-2014, 06:58 PM   #29
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Have you ruled out a leaky injector?
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Old 05-17-2014, 06:17 PM   #30
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Yep

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