Spraying carb cleaner in '89 TB = interesting response

My first post to this forum, so hope I get it right.. I tried posting this yesterday morning, and it never showed up... strange.

I've been reading posts here for awhile and searched on all sorts of difficulties I've encountered in swapping an engine/tranny/transfer case from an '87 Cherokee (Laredo) with Selec-Trac into an '89 Cherokee that had Command-Trac. Both XJs have 4-speed automatics and the 4.0L I6. The swap has been done and the engine runs... at least at idle.

It wouldn't even run until I checked the basics: spark, fuel, air, timing and determined that the fuel pump 2" transfer hose inside the fuel tank was broken. Repaired that, and the engine starts and runs.

However, when I move the accelerator the engine stumbles, coughs, and backfires.

So I've started down the path of troubleshooting. Here's what I've done so far based on advice given to others here at :

-checked timing (distributor rotor points to #1 plug when #1 cylinder is at TDC on compression stroke)

-cleaned and examined cap and rotor

-checked firing order

-removed exhaust including cat and muffler

-checked fuel pressure (33/39)

-checked that all injectors are firing

-changed out 02 sensor

Now I'm at the stage of cleaning the throttle body.

I notice that when the engine is running, and I spray a good shot of carb cleaner into the throttle body and open the throttle, at some point the engine takes off and runs great - rpms go up as high as I want, and I've got great performance as long as I keep that throttle open. Even without adding more carb cleaner. But when I back off on the throttle to idle and then try again without carb cleaner, the coughing, sputtering, and backfiring is there again..

Any idea what might be causing this engine problem?

Seems to run ok once it gets up to a certain point in rpms.

I'm just trying to understand why a temporary application of carb cleaner will allow the engine to run well until I let off the throttle.

Any insights greatly appreciated. -Rick

Could be something elsectrical. Maybe one of the many electrical components won't let the engine get thru to that range where it's running great but the Carb Cleaner will naturally speed the engine speed up so it gets you past it.

There should be a plug coming out of the left side of the throttle body(when looking at it from the nose of the truck) that goes to the MAP Sensor on the firewall, is it connected or cracked? It's a black tube for vacuum.

Zoro said:

Could be something elsectrical. Maybe one of the many electrical components won't let the engine get thru to that range where it's running great but the Carb Cleaner will naturally speed the engine speed up so it gets you past it.

There should be a plug coming out of the left side of the throttle body(when looking at it from the nose of the truck) that goes to the MAP Sensor on the firewall, is it connected or cracked? It's a black tube for vacuum.

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I *did* remove that plug and reinstall it before putting the engine assembly back into the rig. Used some white lithium grease to make it go in easier, and it still has not seated flush against the TB. I think maybe there's still some grease in the top plug hole that's may be preventing it from going all the way in. I'll take that out and clean it up and reinstall. Map sensor appears to function. I pulled vacuum line from it while idling, and engine died. Pulled electrical connection from it, as well, and the engine died again. Haven't done the test per procedures, though. I'll do that soon.

Still wondering why engine will perform so well after it gets to a certain rpm. I've got it on my list to swap out the CPS to see if that might make a difference. I used the CPS from the '87 engine, because the connector looked newer, but maybe it's faulty.

Thanks for the feedback.

My bet is on the throttle position sensor or the MAP is hooked up backwards. The rubber dual plug that fits the hard line to the MAP, into the throttle body can be installed upside down. I cant remember which way it goes right now, I think the line goes in the lower hole.

A quick rough check for the TPS can be performed by starting the engine and let it idle. Reach down and rotate the arm on the TPS, the engine should immediately stumble badly. If it doesn't, the TPS is not sending the position info to the computer. They commonly fail if they get washed or even wet.

What about back pressure? You said you took off the muffler and cat no? I have read that the 4.0L needs a certian ammount of back pressure to operate properly. If you got your RPM's up and built up some good resistance and it went well, then when you bring it back down to low rpm's it gets jacked again... well... it seems reasonable to me. I don't know jack though, so take it for what it's worth, but I would put your exhaust back together and try that again.

The (so-called) Nerd

On my 88 which should be the same the MAP went into the lower plug.
Take out the plug and you should be able to see the open port.
Some previous owner had fixed the MAP line by gluing it in to the rubber grommet, well they effectively glued the tube nearly closed.
I took the grommet out and found it had slit on them also, making the glue job that much worse.
The top of the grommet is just a solid piece of rubber, goes no where that I can see, just holds the whole thing in place or something.
I just grabbed a replacement at the wrecking yard when I was there, many to choose from, I just picked the best looking one.

old_man: thanks, I have the vac line for the Map in the lower hole, and I confirmed that was the open port to the throttle body while I had the engine out. Also, when I disconnect that vac line, the engine dies. I'll test the MAP sensor today. Also will try the procedure you recommend for the TPS - good idea. I swapped the TPS from the original '89 engine over to this engine and that didn't make any difference, but I didn't adjust per specs, yet. Will do. Not sure what you mean by TPS hooked up backwards. I put it on the way it came off. And I think the plug only goes in one way, but I'll confirm that.

TheNerd: appreciate your thought. I've tried with exhaust on and with it off. Also blew out cat and muffler with air hose both ways, shook cat with no rattling sound. Performance didn't change. But that was my thinking, too, initially - symptomatic of a blocked exhaust, until I did the carb cleaner in the TB manuever... when it ran great with a throaty sound of power, I knew my exhaust wasn't blocked.

Langer: my thinking, now, as well.

XJ88User: thanks for confirming that position for the rubber connector to the TB. That's the way I have it.

I'll go out and do the things you all have suggested and will also check the grounds and adjust the TPS and check the MAP. If that fixes it, thanks. If not, I'll be back. Much obliged, Rick

Well people have given a lot of ideas so here's my theory.
The TPS is a variable resistor.

At the idle position, the TPS may be OK. In the area where you start to open the throttle the resistive material on the TPS may be worn or be making bad contact. After you get past that range the contact is good.

This would explain why in the bad range, the engine stumbles. The ECU doesn't know that the throttle is opening so it doesn't increase the fuel coming out of the injectors so it is essentially being starved for fuel. By spraying carb cleaner you compensate for the injectors not responding until the TPS gets to a range where the computer senses the position of the throttle and gives the appropriate amount of fuel.

My guess is a bad TPS. Check the resistance on it as you move it through it's range. If it opens up in the middle somewhere, you've found your problem.

MAP

Well, I've done everything you've suggested and then some to no avail. Here's the list:

-checked grounds -- cleaned to bare metal and reattached using De-Ox-it to prevent corrosion. (Found three ground wires on right side of engine block - two that come from wiring harness and go to dip stick mount and the negative ground from the battery which I attached to another bolt on block just behind distributor. Found two grounds on left side - one that comes from back of engine and the other that comes from the firewall, both attaching to stud on intake manifold.) Are there more that might cause the engine backfire?

-per Map's suggestion, checked risistance of TPS over full range of operation and there were no dead spots - the change was continuous.

-adjusted TPS per FMS: 3.85V output / 4.65V input = 0.83 per specs (note: when I ground directly to throttle body assembly rather than to pin D, I get an input voltage of 5.65V. Should I make a direct ground from that wire and will that resolve the problem?)

-checked torque of intake/exhaust manifold bolts and they were all good

-swapped out MAP sensor and checked flow from line to TB - no change

Then I sprayed the carb cleaner into the TB again while trying to apply more throttle and got the usual response -- engine took off after a bit and was strong and responsive. I could push the throttle open and get a good strong burst of power from the engine, just like it should perform. As long as I didn't let it go back to idle the engine ran great with just gas. When I let it go to idle, it idled fine, but when I open the throttle again without carb cleaner -- cough, sputter, backfire. I still haven't uncovered the cause...

Any thoughts on what I might check next? Thanks, Rick

I still think you should REPLACE the TPS as it may have a dead spot just off idle.

Just out of curiosity, have you triued swapping in the other set of injectors? The ones in there may be firing, but if they are old and were sitting for awhile, they may be partially clogged or varnished up and when they need to spary more to enrichen the mixture as compensation for opening the throttle plate, maybe they won't spary enough fuel.

Does the stumbling and dying happen regardless of how you try to increase from idle speed? In other words, (a) what happens if you suddenly open the throttle pretty wide, compared to (b) what happens if you ease it up from idle very slowly and gradually?

Have you tried swapping the TB?

Tally_XJ said:

I still think you should REPLACE the TPS as it may have a dead spot just off idle.

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You may be right, Tally_XJ. I thought I checked both TPS's carefully - the one that was on the donor '87 and currently in the vehicle and the one that was on the engine that was replaced. Paid particular attention to the region just above idle. I also tried moving the accellerator rapidly to midway in order to determine if that might somehow solve the problem - no. I'll put it on my list to order a new TPS.

Eagle said:

Just out of curiosity, have you triued swapping in the other set of injectors? The ones in there may be firing, but if they are old and were sitting for awhile, they may be partially clogged or varnished up and when they need to spary more to enrichen the mixture as compensation for opening the throttle plate, maybe they won't spary enough fuel.

Does the stumbling and dying happen regardless of how you try to increase from idle speed? In other words, (a) what happens if you suddenly open the throttle pretty wide, compared to (b) what happens if you ease it up from idle very slowly and gradually?

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Eagle, that might be it. The engine from the donor '87 Laredo was removed as one engine/tranny/t-case several years ago and has been stored covered in our garage ever since. Perhaps, when I got it running the other day, all or most injectors began leaking. Maybe that's why all the plugs are fouled, and maybe that's why the carb cleaner causes the combustion to improve to the point where rpms increase and the leaky injectors don't matter.. good thinking. I'll put it on my list to swap out the entire fuel rail from the old '89 engine. Note: both engines ran well when they were last started. Except the '89 engine had bad piston slap and was definitely on its last legs. The '87 engine with only 60K miles on it ran great before I yanked it, but like I said that was quite a few years ago.. Regarding your second and third questions, as I mentioned to Tally_XJ, I tried opening the throttle wide very quickly, and I also tried a gradual opening, as you suggest. Nada.

mrbill3 said:

Have you tried swapping the TB?

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No, mrbill3, but I will.

I really appreciate the help members are providing. It's giving me some good ideas and also keeping me motivated to solve this backfiring problem. Thanks!

Well it certainly looks like the engine runs lean just off idle.
It could be some kind of vacuum leak. Have you tried checking the vacuum with a vacuum gauge?

Like Eagle says it could be dirty injectors but I still suspect the TPS.
Why not throw a bottle of fuel injector cleaner in there. I hear the Valvoline product is pretty good. For example...if you have only a quarter tank, try putting in only 1/4 to 1/3 of the bottle contents and letting the thing idle for a very long while??
You never know.

MAP

Don't think any comments were made regarding your EGR valve. I experienced bad stumble off idle with my 4.0L '88. Disconnecting the small vacumm line running from the EGR solonoid valve to the EGR covered up the problem. The line was disconnected when I bought the Jeep--I noticed it later, and reconnected the line, only to experience the stumble. Seems you've checked every thing else, so insure the EGR system is operating correcly.
The valve senses exhaust back pressure and has a large internal spring to control opening of the valve. I modified this spring (made it weaker) by trial and error until the stumble stopped. I'm too cheep to buy new parts if I can fix them myself!

A 1 volt difference in the grounds is not right and is enough to screw up the sensors. Measure voltage from engine block to fire wall and to the battery negative. If the grounds are good, each should be roughly zero volts.

Did you run the braided ground from the headbolt to the firewall?

Based on the latest iteration of recommendations from old_man, Tally_XJ, Eagle, mrbill3, MAP, and xjbubba here's what I've done:

-ordered new TPS

-ordered a fuel injector o-ring set so I can swap out the injectors with six I know were working just a few weeks ago.

-ordered a throttle body gasket so I can swap out the TB

-poured Seafoam in the gas tank, but don't want to add anything else because I'm concerned that different injector cleaners may not work well together. I will try letting it idle for a long time, though and see if that makes a difference. Previously, I'd just run it for awhile and if I tried to rev it up with the backfiring and all, the cat and muffler would get very hot. Perhaps that won't happen, if I just idle it.. I believe that old_man has suggested a higher-priced cleaner, but I haven't found it in town. I'll look for it online and get some for the next tank of gas..

-checked EGR valve and vacuum seems right and it seems to work - stumbles when opened on idle and seems to have fully open and close performance. I also had it off before installing the engine and cleaned it out well, and made sure the port into the exhaust manifold was clear.

-checked the grounds again. I reconfigured the ground from the rear of the engine so that it went to the firewall (I'd previously joined it with the left side firewall ground at the intake manifold, which I don't think is correct, as the old_man let me know.) I currently have braided ground straps OEM from intake manifold to firewall and from block to firewall. I have regular OEM cable from neg term of battery to engine block and the two wires that come out of the harness and go to the block are insulated wire of fairly heavy gauge (10 or 12?, not sure, but they are OEM).

-checked ground voltages:engine block to firewall: 2mV (neg lead on block); engine block to neg battery terminal -2mV (neg lead on block); firewall to neg bat term -5mV (neg lead on firewall). Is that too much leakage? Should I ground the D terminal of the TPS directly to the block or intake manifold as some members have suggested to others?

These all seem like good ideas.

Any thoughts on what else I might try while waiting around for parts to arrive?... probably a week or so. Thank you, Rick