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Sluggish, Lacking Power


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Okay, my question will take some background first.

 

My MJ just got back on the road after a year restoration.  The engine was completely rebuilt.  Ever since, the engine has felt like it did not have the power it once did.  (I know the 4.0 is not a racer.)  It just felt like it was not firing correctly.  I took it to my mechanic.  He checked the distributor position and timing.  Everything was correct.

 

When I excellerate in 1st, 2nd and 3rd gear, the engine hesitates and chugs as the RPM get above 2000.  I don't experience it in 4th and 5th, but I don't take it up that high, either.

 

On Tuesday this week (high of 10*), I took it in for the emissions testing.  The first attempted errored out.  The tech cleaned his equipment and tried again.  Same result on the test: "Error Low Flow".  That meant the computer was not registering enough gases to calculate a test result.

 

They had me pull up to the next testing station.  Different machine; same results.  (See definition of Insanity.)

 

I did not pass the emissions test, but I did not fail, either. 

 

I got a 5 month extension to fix the problem.  However, they could not tell what the problem was?!?!? :huh???:

 

A tech asked me if the exhaust may be clogged.  That's when a light went off in my head.

 

Several years ago, I had a clogged cat.  The truck felt like I was pulling a battleship behind me.  I could not get up to even 25 mph with the engine chugging.

 

Are my symtoms from a partially clogged exhaust system?  How can I test for clogs?

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Sounds to me like you have answered your own question. :D  The "Low Flow" error should be a dead give away, but just you watch, its gonna make a liar out of me. Every single thing you have listed points directly at a clogged cat, its more common than you might think.

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I've read a few times about a thing that produces similar "clogged cat" symptoms happening with brand new exhaust systems. Basically what happens is that the exhaust tubing has the same (or slightly smaller) exterior diameter as the muffler input/output, so the exhaust tubing sits inside it, but gets pushed in far enough it pretty well hits a baffle inside the muffler, basically blocking off flow. I'm guessing that's probably not what happened, because I've never actually seen a muffler connected that way outside of the internet, but if that's the way yours is hooked up, it can't hurt to check.

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When I first got my truck, it would overheat going up long grades maybe 1/4 mile long. On level ground it would run fine. Went through the typical cooling system replacements; radiator, water pump, tranny cooler, etc. with no changes. I had installed a new header and brought it to my friend's muffler for a dual exhaust install. When he cut the pipe off the front of the cat, you could look into it and see a black, gummy tar-like substance covering the cat honeycomb. I don't see how it ran at all. Finished up the exhaust, replaced the cat with a resonator, no more overheating and it ran great.

 

I've never seen a procedure for flow-testing an exhaust system in the FSM.  :dunno:   Does your truck get hot climbing up long grades? I'd bring it to a muffler shop, have them cut off the cat inlet pipe, and take a peek inside. Replace the cat if there is any doubt.

 

EDIT: Another thought: were any modifications made to the engine during the rebuild? Like an HO head on your Renix block causing exhaust port blockage?

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  • 2 weeks later...
CRUISER’S VACUUM TEST FOR EXHAUST RESTRICTION

 

Your vacuum gauge should come with an instruction booklet outlining the procedure. Hook the vacuum gauge up to a source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum. Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle. Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction. 
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87MJTIM - I'm sure you have your reasons but in Maryland your truck can be registered with Historic tags thus bypassing all the emissions testing crap and you can ditch the cat.

 

Yeah, I know, but it is my DD.  The historic tags restrict the use of the truck.

 

I know people who have ignored it and drive them regularly.  I also know people who have been stopped and told to re-register the tags.

 

I just got an 84 CJ7 that I will register historic.

 

BTW - Have I seen you driving around here?

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CRUISER’S VACUUM TEST FOR EXHAUST RESTRICTION
 
Your vacuum gauge should come with an instruction booklet outlining the procedure. Hook the vacuum gauge up to a source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum. Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle. Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction. 

 

Thanks for that info. 

 

My mechanic will test the temp going to and out of the cat.  He will use a infra-red temp gauge.  He said the cat will heat up the exhaust fumes.  The readings should be hotter on the back end.  If it is not, then the cat is blocked.  (Or is he just blowing smoke?)

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87MJTIM - I'm sure you have your reasons but in Maryland your truck can be registered with Historic tags thus bypassing all the emissions testing crap and you can ditch the cat.

 

Yeah, I know, but it is my DD.  The historic tags restrict the use of the truck.

 

I know people who have ignored it and drive them regularly.  I also know people who have been stopped and told to re-register the tags.

 

I just got an 84 CJ7 that I will register historic.

 

BTW - Have I seen you driving around here?

 

 

 

My tags are due next month. I went to the state website yesterday to see what the requirement is for antique tags in TN, you are right, driving is restricted to weekends and events. Antique tags are one time fee, that's why the restriction, permanent tag.

 

In TN there is a second option, they will issue 'vintage' tags up to year '88.....may want to look at that.

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CRUISER’S VACUUM TEST FOR EXHAUST RESTRICTION
 
Your vacuum gauge should come with an instruction booklet outlining the procedure. Hook the vacuum gauge up to a source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum. Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle. Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction. 

 

Thanks for that info. 

 

My mechanic will test the temp going to and out of the cat.  He will use a infra-red temp gauge.  He said the cat will heat up the exhaust fumes.  The readings should be hotter on the back end.  If it is not, then the cat is blocked.  (Or is he just blowing smoke?)

 

 

That's a quick and easy way to do it. 

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