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Discussion Starter · #1 ·
I have a 91 YJ that I am working on. Over the last couple months I swapped long blocks due to the previous engine being worn out and a rod knocking. It barely ran when I picked it up. Now I have finished the engine swap and it still runs just as badly. I used all the sensors from the original engine so I really doubt it is the new engine. It doesn't give me any error codes except 12 and 55 which is battery disconnected and done with codes. It only runs with the gas pedal about halfway down and backfires through the intake a bit. It is acting like the timing is way too far advanced but it isn't adjustable. The previous engine was blowing a lot of oil back into the intake from the valve cover so I changed the MAT sensor thinking it could be like the problem with the K&N filters. It was covered in oil but didn't make any difference. Any body have ideas what I can test next?
 

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are your spark plug wires on the right spark plugs? maybe one or two are mixed up?



and this might be kind of a stretch....but what about the throttle body. theres a little motor on the side that adjusts for idle speeds...if its really fouled up that might not let the engine idle very well.
 

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are you sure you aren't a tooth off on the distributer?
try taking the bolt out and turing the distributer further than the bolt allows.
On my stroker I had to grind the tab off and use an older style clamp from a 5.0 mustang.
 

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Will, if this is the SAME symptoms as the other engine.... look for perhaps a severely constricted exhaust. Try the simple things first...
 

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You might want to check your O2 sensor. If you were burning that much oil it is probably fouled with carbon. If it is, you'll be able to see it visually.

That old engine with blow-by and a rod knocking should have run fine other than the loss of some power and the loss of oil, well then there's the noise too. The new engine runs the same way because you transferred the problem to it.

I had problems with my 'puter controlled TPI sbc. I bought a real time scanner/monitor for it. Knowing what sensor is giving what reading is a huge help. The scanners are expensive but not compared to the alternative of swapping parts until you get it right.

It reminds me of the good old days with vacuum tube TV sets. When the TV went on the fritz, people took their tubes out to test them. There were tube testers all over the city in those businesses that sold tubes. Most often, none of the tubes really tested bad so they would buy replacements for the ones that looked like they might be marginal or somebody advised, "When my set did that it was the XXXXXX." Eventually, they had accumulated a box full of questionable tubes, which they got out and started using as replacements every time the set went on the fritz again until they either got a suitable picture and sound or went out and bought more tubes.

I think a scanner will save you money in the long run and I know it will save you a lot of grief. In my particular case the engine was running rich and about like yours. The problem was a coolant temp sensor that was reporting 38 degrees below. It doesn't throw a fault code until it reaches minus 40. I had Precision Tune hook it up to their scanner and they missed it, just charged me a little more than the price the scanner would have cost had I bought it in the first place. So, I still had to buy a scanner and found a $10 part bad. Since then, the scanner has found problems for me that were free to fix, a bad or dirty connection.
 

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Discussion Starter · #6 ·
Thanks for the ideas. Spark plug wires are correct. Spark plugs are changed. The previous engine was already going to be swapped due to the blow by and knocking. It was only a week before I started on it that it started to not run.

I cleaned out the main bore of the throttle body but the idle circuit has tamper resistant torx screws. I am not surprised it won't idle but it is running about 1000 rpm with the throttle about 3/4 open.

As for being off a tooth on the distributor, I could be. I thought the timing was based on the crank position sensor and the distributor sync sensor was just to tell it it was turning. Still, it wouldn't explain why the problem showed up before I pulled the engine.

Plugged exhaust was one of my first thoughts. One of the times I got it to start, my dad checked the exhaust and said it was not acting plugged. (He's the one I learned from and has probably forgotten more than I'll ever learn.)

The O2 sensor shouldn't come into play with a cold engine. The longest I have been able to keep the engine running so far has been about 30-45 seconds and only about 3-5 times each day the last two days.

I'll have to look into a scanner.
 

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Will I'm surprised at you.

The scanner may help -- but try cheap first!

Fuel or ignition?

No matter how complicated the computer system, it still boils down to the basics! Compression/Air/fuel/spark/and when.


Add propane to see if it's fuel starvation. That will eliminate - or verify fuel related. Fuel is mostly computer related. Once you know it's a lack of fuel you can go from there.
Timing light will tell you the ignition is firing at the right time. Also computer related.

No help? Ignition trouble, rotor etc., exhaust, battery voltage etc.

Even when diagnosing - KISS.

Test, isolate, test, isolate - verify, fix.
 

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[ QUOTE ]
The longest I have been able to keep the engine running so far has been about 30-45 seconds and only about 3-5 times each day the last two days.

[/ QUOTE ]

I think you left that part out the first time. That is a totally different situation.

With a bad vacuum leak or very wrong ignition timing, it would still keep running but like you said, you may have to keep the throttle at half-open or better.

Is it hard or easy to start? This may help.

My guess would be fuel. The next time you get it started, when it dies, pull a couple of plugs and see if they are fuel fouled. If you have or can get a fuel pressure gauge, hook it up and see if you have good pressure and if it drops as it dies.

I ain't seeing it as ignition. Way off timing would keep running but poorly. 30-45 seconds isn't really long enough for a bad ignition component to heat up but maybe.
 

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[ QUOTE ]
Will, if this is the SAME symptoms as the other engine.... look for perhaps a severely constricted exhaust. Try the simple things first...

[/ QUOTE ]Yep ssounds like a plugged cat. Pull the o2 and see if it runs any better. Free and easy to eliminate it.
 

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Will, I'm wondering if you're not describing a miss in the engine... and that's because the fuel pump is not supplying adequate fuel pressure.
 

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"""""I ain’t seeing it as ignition. Way off timing would keep running but poorly. 30-45 seconds isn’t really long enough for a bad ignition component to heat up but maybe. """""

Not so much as a heating up compoent -
Something as simple as a bad coil wire can do exactly that. When cranking a little extra fuel gets in, required voltage is lower. As it runs a moment, and the computer starts to lean it out a little, required voltage rises a little.
Worth a try anyway.

Most all the rest of the suggestions come back to fuel - or lack of it. Propane test takes about 10 seconds to prove or disprove it.
Or, spray a touch of starting fluid down it - it's same as fuel - but be careful, if it backfires the can could explode. Be sure to wear safety goggles and keep your face away.
 

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[ QUOTE ]
Not so much as a heating up compoent -
Something as simple as a bad coil wire can do exactly that. When cranking a little extra fuel gets in, required voltage is lower. As it runs a moment, and the computer starts to lean it out a little, required voltage rises a little.

[/ QUOTE ]

Yeah, that is part of why I asked if it was hard to start.
 

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Discussion Starter · #13 ·
I haven't had a chance to work on it since posting this but it has been very hard to start too. I have to crank it for about 15-20 seconds for it to catch.
 

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Check your spark. You need a nice blue one. Yellow won't make it. Then check your fuel pressure. Might not be a good idea to do these in reverse if you spill any fuel.
 

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Years ago I had a VW baja bug and it ran fine. One day it started acting the way you are discribing. After several weeks of trouble-shooting and finaly taking it to a shop...still didnt fix the problem. I stumbled accross the issue. My aftermarket Tach went bad. I unhooked it and solved the problem. reconect it the problem came back. Look to anything that you have added to the jeep prior to the problem starting. good luck.
 

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[ QUOTE ]
Years ago I had a VW baja bug and it ran fine.

[/ QUOTE ]

Not to hijack the thread, but I saw one of those running down the highway last week. Looked to be in good shape too considering how old it would have been.

Biggest problems I saw with a VW was dirt buildup on the plug boots letting the spark ark to the shroud or the air cooling fan blowing them out of the holes.

Today, I saw both '56 and '57 Chevy convertibles a couple of miles apart on that same highway. Same highway I saw a guy riding his 'cycle at about 80 per ON THE REAR WHEEL ONLY.
 

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Discussion Starter · #17 ·
After thinking a bit more about it, my Jeep started to run similar when the throttle position sensor went bad. I still haven't done anything with this YJ though. It is too darn hot outside still.
 

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Discussion Starter · #18 ·
I worked on it a bit last week. I changed the MAT sensor since the old one was full of oil. I changed the TPS but that didn't fix it either. I pulled the other sensor wires to see if I could get it to a default program that would bypass the problem sensor but still no luck. I am pretty stumped on this one. It still barely runs with the throttle about 1/2 open (doesn't run any other way). It also backfires out the intake sometimes but not consistently.
 

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So what did adding propane do? (That's the first thing I'd do long before throwing parts at it.)

And - try cranking it while watching a timing light on the mark. See if it's double triggering, not triggering enough or way out of time.

Compression, air, fuel, and spark at the right time - still the basics no matter how much electronics it has. Sometimes we end up chasing our tail - we make it complicated!

Plugs are clean? Oil in the intake may have them really fouled.
An old tow truck driver's trick that works sometimes to clear fouled plugs: Pull the coil wire out slightly, either end - make the spark jump about 1/4". The extra gap gets the firing voltage up higher on the curve - where it's rising faster. If it starts then, leave it out awhile to let the plugs get hot and burn away the crud.
Works great on gas flooded engines, sometimes it can clear out oil too.
The principle is the same as the "Spark Intensifiers" sold at county fairs and JC Whitney.
 

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First time reading this thread. I don't see that you have checked fuel pressure. I'd check it. (I already have a gauge) If you can't check it try Rrich's propane suggestion, if that helps. Check fuel pressure. Have you changed the fuel filter?
 
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