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89 2.5 runs but backfires into TBI after about five minutes

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So here's what I have;

89 2wd Pioneer 2.5 (4 cyl) Renix with 230 k on the odometer. Its only doing local runs down the block and back to the casa because of its running issue.


My issue:

It cranks and idles and runs fine. But after three to five minutes the engine (when under load) will miss, sometimes it will miss 7-8 times, (it sounds like tack-tack-tack-tack).

When the fuel gets back to the cylinders (while under load) it runs fine for a few minutes or maybe one or two minutes, or maybe just a few seconds, but when you try to accelerate it often misses again.

When I let it idle it runs fine (it will do so for however long I let it run), but when its warm I can give it gas and 1 of every three times (guestimate) it will miss and sounds like it is backfiring into the tbi.

I took it to my mechanic who did a compression test and told me that my #1cylinder has an exhaust leak- he recommended that I find a newer/new engine and transplant it or remove the valve cover and try and free up the exhaust valve. He did not recommend replacing the cylinder head due to the mileage on the engine.


My situation

I am in the process of getting a newer (4 cyl out of an 87 manche)engine sent to me. I wanted to make sure before I put in a newer engine that my fuel pump is up to par and not the cause my problem, or going to cause me additional problems.

I plan on doing a pressure test on the fuel pump (whenever the parts store gets a fuel pressure gauge in ). I think that the timing chain needs to be redone and the emissions light is on so the 02 sensor is likely not helping things out.

When the truck wasn't doing this it was getting about 33 MPG (with the emission light on). I think it could be something clogging the fuel filter as well (Probably in tank).


So comanche gurus I ask for your wisdom and guidance. What do you think I should do to get my manche healthy again? Anybody else have these issues?


Thank in advance for the replies

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I don't quite follow you............You want to get your 2.5 up and running, but you have another one on the way :hmm:


With 230K on the clock, I would think the engine is about used up :dunno:


I don't know 2.5 all that well.......but generally, from what you write, with the engine mis-firing, under acceleration, and the back firing, that's a sure sign the timing chain is shot or stretched, and If I remember right, the 2.5 also has a slack adjuster on the timing chain that wears out.


It your CO2 sensor is shot, and the CEL is on, it's not telling the ECU to operate in the "closed loop" and it's staying in the "open loop" operating mode. It's kind of hard to believe that your getting 33 mpg operating in the open loop mode :hmm:


Your heading in the right direction to see if your fuel pump is running correctly by doing a pressure check, but you can pick up a new pressure checker at HF for $10 which is good enough for a couple checks.

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Thanks for the reply,


What I am trying to do is make sure when I install the newer engine that the fuel pump isn't going to cause me a headache or possibly some other problem that I did not recognize. With these trucks its hard to say sometimes.


I know its foolish to try and second guess what may occur with a transplant but I wanted to make it as clean as possible when it happens. Sometimes it pays to ask around.


Yeah I know its used up at 230k but it was doing that good on gas before it started stuttering, one of the reasons I love the little truck. Maybe its the gearing who knows. Your right about the timing chain adjuster.


I am considering having the 2.5 that I am taking out rebuilt, that is why I was talking about timing chain replacement stuff. Didn't mean to confuse.

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