87 mj won't rev.

[JeepcoMJ]

Working on my room mate's 87 mj.

 

4.0 aw4 4x4.

 

 

It wouldn't rev past 3600. I initially thought cps, or ecu. Swapped both to no avail. New cps actually makes it not rev past 3000.

 

New distributor, properly indexed...no improvement.

 

I've swapped the throttle body, map sensor, ignition coil, and the sensor on the intake manifold with no improvement.

 

 

Fuel pressure at the rail is max 20psi with key on, 10 when running.

 

Replaced all vacuum lines because they needed replacing, and I figured it couldn't hurt.

 

Compression is 120, so it's not a timing issue.

 

This truck was subject to the cps recall and has had the c101 bypass done, where the wires go directly to the ecu rather than through the loom. Voltage test shows 0 ohms on both wires, so that's not the issue.

 

My last bet is the fuel pressure regulator. That fuel pressure is way too low, but I don't know what it's supposed to run at off the top of my head.

 

 

The truck revs just fine from idle to 3k, then bounces like it's on a rev limiter.

 

Last thing is that the exhaust manifold is cracked pretty bad. The 02 sensor is junk. But that cannot be the cause of the issue...the truck should rev fine in bypass on the ecu until it's warmed up, at least.

 

Anyone smarter than me? ...cause this has got me stumped.

 

Thanks,

Pat

[JeepcoMJ]

Oh, and the cat isn't clogged...there isn't one.

[Wildman]

I'm Not Smarter that you :dunce:

 

But, the fuel pressure is too low for the 4.0 :D

 

It should be 31psi with the vacuum connected to the pressure regulator, (key on) and 39psi with the vacuum disconnected.

 

It could be a bad fuel pump, or a clogged filter, either the frame filter, or the in-tank sock filter.

 

Also........others' have had problems with the soft line in the tank from the pump to the sender units hard line, leaking, cracked, fallen off, or no hose clamps on that soft line.

 

Yes, it's worth pulling the sending unit out to double check everything before a new pump is ordered :yes:

 

And.......if you want the complete run down on testing the fuel pump -

 

http://www.lunghd.com/Tech_Articles/Fue ... ostics.htm

[JeepcoMJ]

thanks Wahoo. and you probably are smarter than me.

 

 

I accidently deleted the part about it having a new fuel pump, sock, hoses from pump to hard lines, vent lines, gas tank, and fuel filter.

 

 

 

so I've ruled those out. I think the only thing it could be, now, is the fuel pressure regulator...thinnking of just swapping the whole fuel rail and injectors.

[Wildman]

( It wasn't Wahoo whom posted) :roll:

 

Even with a new pump, still don't mean a brand new pump is not the problem :shake:

 

Mvusse went thru 2-3 brand new pumps a couple years ago just to get one that would work correctly.

[JeepcoMJ]

sorry, i saw last post as wahoo. just woke up, all over the place and my head isn't screwed on yet.

[JeepcoMJ]

( It wasn't Wahoo whom posted) :roll:

 

Even with a new pump, still don't mean a brand new pump is not the problem :shake:

 

Mvusse went thru 2-3 brand new pumps a couple years ago just to get one that would work correctly.

 

 

I should have said "different pump". I took the pump and sender out of another 4.0/swb mj that had no running issues and had roughly 30psi, and put it in the truck.

 

 

I don't spend money on parts if I can avoid it lol.

[JeepcoMJ]

ruling out fuel pressure as the issue.

 

 

my pressure gauge apparently reads low. 89eliminator's truck shows the same identical readings as this one.

 

 

so. I'm going to just swap the fuel rail for $#!&s and giggles, and look at the plugs, wires, and cylinders better. if none of this works, I'm telling the room mate to scrap it or put a new motor in it. I'm not doing anything serious on a POS truck, though.

[jpdocdave]

I was gonna say, that fuel pressure is for sure the problem, but not if your gauge is not right. and before that i was gonna say cat, but you already cleared that from the equation.

[Wildman]

Ok.........

 

But, if your going to swap out the fuel rail, why not also swap out the injectors :idea:

 

Could be a plugged/faulty injector in there too :brows:

 

I'm sure you have 6 or 8 sets laying around :roll:

[JeepcoMJ]

Lol. I just assumed that that it would be assumed that I'd do that, too

[jpdocdave]

I just read this a little closer, I have a rule in diagnostics, don't overlook or dismiss known problems by theorizing why thats not it, fix known problems. Its called the basics. How can you be sure a leaking manifold, and bad 02 isn't causing this, my mj acted all sorts of stupid with a bad o2, and right off the bat, not after warming up.

 

I assume the throttle body you tried, means the tps was also swapped, and no pinched fuel line anywhere.

[JeepcoMJ]

Went against my opinion and replaced the o2 sensor with a brand new one. Like I thought, not the issue.

 

Knock sensor is broken, tho.

[Wildman]

That might be your problem :idea:

 

The knock sensor sends a signal to the ECU to retard the timing when a knocking condition exists, that could be tossing off your timing on high rev, lack of power

 

Better have a good one in your parts collection........they run about $70 :eek: