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Old 07-15-2008, 07:13 PM   #1 (permalink)
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Runs great cold, lousy after warm-up

I just replaced the engine (original = 180k w/ thrown timing chain and worn out, new = 115k w/ unknown history) on my 94 5-spd 4x4.

Everything went pretty well until the very end. I set the timing on the cold engine and it ran very smooth. After warm-up, however, it runs rough and barely idles, sometimes stalling. If I let the truck sit and cool off, it runs well again until it warms up.

Plugs (gapped), distributor cap, rotor, exhaust manifold gasket are all new. I checked and adjusted all the valves.

I have swapped several parts from the original engine onto the new one with the same bad performance: throttle body (w/ TPS) and two coolant sensors on front of intake manifold. I also listened to the injectors with my stethescope and they all sound fine.

I get no CEL.

A helpful Yotatech member has suggested a bad EGR valve.

Any other ideas or suggestions?
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Old 07-15-2008, 07:20 PM   #2 (permalink)
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Quote:
Originally Posted by d-cubed View Post
I set the timing on the cold engine and it ran very smooth. After warm-up, however, it runs rough and barely idles, sometimes stalling. If I let the truck sit and cool off, it runs well again until it warms up.
You gotta set the timing with the engine hot!!

Which motor, 3.0 or 2.4?
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Old 07-15-2008, 08:26 PM   #3 (permalink)
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22RE

The engine runs like crap hot, regardless of the timing. I can't get a stable engine when it's hot regardless of the timing.
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Old 07-16-2008, 10:57 AM   #4 (permalink)
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Did you pull the codes from the ECU? The manual explains it in detail but all you need to do is jump the same terminals you used to adjust the timing, turn the key on and count the flashes on from the CEL. Then compare the numbers to the ones in the book or post them on here to identify what they are.

If you already did the above and you think the timing is at least close, try disconnecting the O2 sensor and see how she runs. You may have a bad O2 sensor. You will get a CEL at that point. There is a testing procedure for that in the manual too. I have a bad one right now and disconnected it and it seems to run fine but is a little rich. From what I understand, the O2 sensor does not operate until the engine is warmed up. As a result, if it's bad, it will have no effect until the engine is hot then runs poorly.

If that isn't the issue, then it could be a bad TPS. Again, consult the manual for the testing procedure. It is fairly involved but is pretty easy to do with a multimeter. And again, you may be able to disconnect it and see how it runs. I suspect the O2 sensor though. There are other sensors it could be as well like the coolant temp switch but I would start there.

Good luck.
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Old 07-16-2008, 11:48 AM   #5 (permalink)
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I shorted the connector and got no codes (constant blinking light).

I swapped the entire original throttle body onto the new engine, and the problem still persisted, so I doubt that the TPS is bad.

The truck ran well before I changed the engine, so I am suspicious that the O2 sensor is bad. I did take it off and probably banged it around a little, so I guess it could have gone bad during the engine replacement.

I will try to run the truck with the O2 sensor removed.
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Old 07-16-2008, 10:26 PM   #6 (permalink)
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I unplugged the O2 sensor. Sure enoug I got a code, but no real change in the idle. I reconnected the O2 sensor, cleared the code, and the truck ran poorly, just as before.

I also swapped in the original distributor, that made no difference.

Things tried so far:
remove O2 sensor
swap in original coolant sensors
swap in original distributor
swap in original throttle body

I have not yet swapped in the original EGR valve.

Any other ideas would be appreciated.
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Old 07-18-2008, 11:35 AM   #7 (permalink)
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There are only a few things that run differently when cold or hot that would make a significant difference in performance. One of those is the EGR and the other is the entire Throttle body assembly. Perhaps the EGT sensor, but I have notice the EGT sensor has little impact.

I would replace or bypass the EGR and test it again.
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Old 07-18-2008, 11:59 AM   #8 (permalink)
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Another member also suggested replacing the EGR valve.

I already tried the EGT sensor (much easier to swap than the EGR valve) and it did not help (as you thought).

I will try to replace the EGR valve itself. I have avoided this step because it's kind of a pain to get to and I will also likely wreck the gasket (one piece intake manifold gasket that is extended rearward past cyl #4 to accomodate the EGR valve).
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Old 07-18-2008, 03:59 PM   #9 (permalink)
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^Are you sure that all the parts you are using from the old motor are good?
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Old 07-18-2008, 04:12 PM   #10 (permalink)
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The old engine ran fine up until the moment I threw the timing chain.......
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Old 07-18-2008, 04:24 PM   #11 (permalink)
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Checked your idle speed adjustment?
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Old 07-18-2008, 09:47 PM   #12 (permalink)
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does it run poorly above an idle?

Or does it just Idle poorly?


any Vacuum leaks?

Disconnect the vacuum hose from the EGR and see if that changes anything
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Last edited by Vicoor; 07-18-2008 at 09:49 PM.
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Old 07-20-2008, 10:30 AM   #13 (permalink)
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I did not check my idle speed adjustment; I assumed that it was good when set on the old engine. However, to keep it idling after warm-up. I did open the idle speed screw one turn. Now it idles a bit fast when cold.

I did pull the vacuum line off the EGR; it made no difference.
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Old 07-20-2008, 12:48 PM   #14 (permalink)
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The egr valve gets a vacuum signal proportional to load only after the engine is warm.

If an EGR valve hangs open then it would idle bad or stall all the time it is hung open It would have nothing to do with whether it were cold or hot.

It should idle fast when cold and gradually decrease during warm up til it gets to full warm.

So, Does it run poorly all the time (even while you are driving done the road) Or does it just run rough at idle?
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Old 07-20-2008, 05:53 PM   #15 (permalink)
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^Ya, when I start my truck dead cold it ildes at about 1600-1700 and then drops to about 900 when fully warm.
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