1989 Jeep Cherokee, 4.0 6cyl, All stock
New to this forum. Have been battling this problem for several months.

Engine sounds great at idle. Revs nicely when parked and sounds good reving up and down.
Start driving and engine sounds fine and has nice peppy start, until you actually try to accelerate or put any load against the engine.
It then falls on it's face, like it's being choked out. I can hear slight backfiring in the intake. vroooooom..... bwaaaaaaaaaaaaaaa pop bwaaaaaa pop pop bwaaaaaa
Seems to get worse the hotter the engine gets. Engine never dies, just bogs down when you get on it to accelerate

Have done and replaced:
Refreshed all grounds
Replace CPS Sensor
Replace Oxygen Sensor
Replace TPS Sensor
Replace Throttle Position Cable to Transmission
Replace MAP Sensor
Replace EGR Valve
Cleaned IAC
Cut out Catalytic
Replaced muffler
Mechanic Shop tested Fuel Pressure both at idle and driving during problem, pressure good
New Air Filter
New Fuel Filter
New Fuel injectors
New Distributor Cap / Rotor / Wires / Spark Plugs
New Coil
Negative Battery Cable

I am out of Ideas on this one. Have followed most every steps and guides online for sensor refreshing and testing ect. many I found on this forum.
And after months of messing with it and replacing parts I finally took it to a shop. My mechanic had it for 2 months and couldn't figure it out.
Any ideas would be much appreciated. Thanks!

 

It sounds like a fuel issue to me. And backfiring in the intake is often related to fuel delivery.

Keep in mind that you can have proper fuel pressure but not enough volume if you have a weak fuel pump. Do you have 31 psi of fuel pressure at idle? Then disconnect the vacuum line leading to the fuel pressure regulator under the hood and fuel pressure should jump to about 39 psi. If there is any raw fuel in the vacuum line leading to the FPR, the FPR has a ruptured diaphragm and it has to be replaced. But know that fuel volume and pressure don't always go hand in hand. There is likely a spec somewhere of fuel volume expected and how to properly test it.

How many miles on this engine? I don't think it is related but you might also want to run a compression test on all cylinders. Good snapshot in time of the internal condition of the engine and you don't want to chase your tail here by not testing it as it's pretty easy to do on this engine. The compression test spec is 120-150 psi with no more than a 30 psi variation between cylinders.

Good luck and keep us updated! I'm sure Cruiser54 will have an idea or two for you to chase!

 

The previous owner had replaced the fuel pump and the regulator a few years ago. I wasn't really chasing those as they still look new. That doesn't mean parts can't be bad or already failed.
I don't have the fuel pressure gauge here. So at an idle when I pulled the vacuum line and it had hard suction on it. There was no fuel or moisture of any kind in the vacuum line.
But for a test I just pulled the vacuum line off the regulator and took it for a quick drive. It ran excellent!

To really know I would need to run it longer until it was completely warmed up. What are the implications of running and driving it with the regulator vacuum line disconnected and plugged?

 

The vauuk;m regulator allows the regulator to increase the fuel pressure a little for heavy demand. But to be sure of what you are doing I would suggest hooking up a fuel pressure gauge

 

Sorry to be blunt, but this makes me question more about your other previous efforts.

When said you "replaced" many of the sensors, were they new? OEM? Aftermarket? Salvaged? After MANY other threads about sourcing the EGR, I know how difficult they are to find.

 

Fuel pressure tested at mechanic shop. I don't have the equipment here. I will probably go borrow one today to help troubleshooting. Pressure would move from 30psi to 38/39psi when disconnecting the vacuum line.

This hasn't ran right since I've owned it. Much of the previous efforts were before this issue. I have worked through many of the issues, missing/stalling/ect and have cured most of them.
The engine actually idles and sounds better than ever, was a daily driver. But then it started developing this lag in accelerating which has turned into it falling on it's face and no acceleration.

Replaced all of those sensors with NEW sensors. Aftermarket, napa/carquest. Have retested most of the sensors and are in spec according to test procedures found on cruiser54 and posts I've seen on this forum.
Yes. NEW EGR valve replaced about a year ago. Don't remember if it was napa or carquest. Sounds like I should have saved my old one.

With the vacuum line off and plugged would imply that it's just running at the higher 39 psi all the time. It did seem to run much better this way. Could the vacuum not be releasing the regulator when in hard acceleration?
I would like to run it this way until fully warmed up and see if problem comes back. Is there any reason I can't run it with the vacuum line off and plugged for testing?

 

265,000 miles Haven't done any compression tests on it.
Smokes a little when first started but otherwise doesn't use oil and is a decent runner.

I agree that it's a fuel issue. Have ordered another pressure regulator.

 

I have seen a weak coil cause a problem like this it will idle and rev good but doesn't have enough spark for a load and it can cause a back fire .

 

Note from DJ
I see no mention of replacing the fuel filter. A restricted fuel filter that is plugged with junked will allow normal fuel pressure at idle but diminish the amount of flow under higher demand. This a cheap maintenance item.

 

The fuel filter has been changed.
I changed the coil (forgot to add to list) when I did the cap and rotor. And I cleaned up the coil and ICU connections when I had it apart per cruiser54 tips.
Sorry for the confusion. I worked on this all last summer, then the mechanic shop has had this for the last (almost) 3 months. Trying to remember all the things we have done to it.

I may look over a few of these things again anyway while waiting for pressure regulator. Not uncommon to have bad "new" parts. Especially aftermarket parts.

Couple of questions on the regulator.
What is the benefit of reducing the fuel pressure? Is it for fuel economy? Why couldn't you just run with it at 39psi all of the time? (Not that I want to do that) Just thinking for troubleshooting right now.
And also, what controls the vacuum going to the fuel pressure regulator?

 

I should have been more clear. It is 39 psi at idle. The vacuum allows the fuel pressure to increase for more spray under load. I think I would be looking at the regulator at this point

 

Timing off "a tooth", either at the timing chain or the distributor

 

You said fuel pressure seemed fine while driving, but try bypassing the fuel pump ballast resistor, or just go ahead and eliminate it by connecting the wires together. Later years don't have it. Also hit cruiser54.com and follow his tips on refreshing your ground points.

 

Just because you replaced all your sensors with new does not mean they are all good.
I know.
I just went through all of these problems with my new engine in my Jeep.
You really need to test all of them according to the factory manuals specifications. That is how I came to find all these bad new parts.

But what you are describing sounds like a bad MAP sensor, or a fuel pump problem?

After I replaced the bad new sensors, my Jeep was running pretty good, but still felt like it was running out of fuel.
I have a fuel pressure gauge, and I hooked it up a second time, and sure enough it was not getting good pressure.
Dropped the tank, and my strainer had fallen off the pump assembly, and the pump sucked up some crap from the bottom of the tank.

So that is all I have to offer, but just because you replaced old parts with new does not mean anything anymore in todays world.

 

The purpose of the vacuum compensated regulator is to keep a constant pressure drop across the fuel injector. That way, a given injector pulse length puts out the same amount of fuel regardless of engine load of vacuum. Later years did away with the vacuum regulator and have a constant pressure fuel rail, and the ECU adjusts the pulse lengths to compensate for the vacuum as read by the map sensor. There are pros/cons to either way. Constant pressure eliminates some plumbing, but it also means the pressure drop across and the spray pattern from the injector isn't consistent.