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Discussion Starter · #1 ·
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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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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Did it do all this BEFORE you rebuilt it?
 

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Discussion Starter · #4 ·
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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Discussion Starter · #5 ·
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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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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Discussion Starter · #8 ·
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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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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Discussion Starter · #10 ·
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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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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Discussion Starter · #12 ·
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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Heat it up again and try to move the screw it will come out.
 

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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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Discussion Starter · #16 ·
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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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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Discussion Starter · #18 ·
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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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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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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