88 4.0, bad fuel pump or something else?

I recently decided to do a small overhaul on my jeep's fuel system due to sluggish performance. I changed the fuel filter, the old one dumped a kinda murky brown fuel out when i got it out. Not as bad or as much as i expected... then i did an injector change. All were oem spec injectors. I noticed my injector ports were HEAVILY clogged with junk so i cleaned them. Well about a week after the fuel filter and injector change, it started to make a knock, not a tap or tick.. but a knock at idle. Not one that sounds like its going to blow a crater in the block, but a noticeable knock. Only at idle and only after driving for a little while. When given gas, it goes away. When its a cold start, its non existing. After driving for like 15-20 mins, ill come to a stop and hear a knock on one cylinder, cyl. 4 i believe. I checked injector plugs and theyre all connected. My best guess is its the original fuel pump and its going out. I think its fine when its cold then when the motor in it warms up, it loses its prime causing low fuel pressure and poor performance. No, i havent checked the fuel pressure on a cold start and warm, i dont have the tool to do it. I could go rent one at a parts store though, but even if i do.. i dont know what im looking for, fuel pressure wise. What should the fuel pressure be? Does this sound like a bad fuel pump? Any help is appreciated!

P.s. ive been driving it like this for about a month or 2 with no change. I tried a different weight oil and lucas additive. No change.

Fuel pressure at the fuel rail test port should be about 29-31 at idle, and about or near 39 at WOT... when warm. There is a thermal vacuum switch in the top front of the intake manifold that, when warm, allows vacuum to influence fuel rail pressure. However, if it was a fuel pump issue, it wouldn't be specific to one cylinder.

Are you sure it isn't a mechanical issue with valves or rods you're hearing?

The only way fuel could be causing your knock is if it were leaking into the engine oil. After the knock starts, pull the dipstick and smell the oil on the end. If it smells like gas, you have a leak.

You may have loosened crap into the intake that got into the combustion chamber cleaning the injector intake inlet areas that mucked things up.

Next time you hear the noise, put it in park and run the engine up to 3500 rpm for about 30 seconds and see if the noise goes away. It will free up sticking lifters.

MMO is a great oil additive to free up and clean sticking lifters.

It is not a fuel pump or fuel issue, it has nothing to do with the fuel pump or fuel. Most likely a lifter sticking, very common on jeeps to make that noise.

Did the performance return after the fuel and injector work?

Hypoid said:

The only way fuel could be causing your knock is if it were leaking into the engine oil. After the knock starts, pull the dipstick and smell the oil on the end. If it smells like gas, you have a leak.

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It doesnt smell like gas, just oil.

Ecomike said:

You may have loosened crap into the intake that got into the combustion chamber cleaning the injector intake inlet areas that mucked things up.

Next time you hear the noise, put it in park and run the engine up to 3500 rpm for about 30 seconds and see if the noise goes away. It will free up sticking lifters.

MMO is a great oil additive to free up and clean sticking lifters.

It is not a fuel pump or fuel issue, it has nothing to do with the fuel pump or fuel. Most likely a lifter sticking, very common on jeeps to make that noise.

Did the performance return after the fuel and injector work?

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The knock instantly goes away at roughly 800rpms and higher. Its ONLY at idle, it doesnt get faster or louder as you give it throttle. And yes, some performance returned after the injectors were replaced. Its not a huge difference though since i was running street tires before all this and now im on 35s and stock gears (which will be swapped soon) so things are sluggish as expected with that kind of setup.

"The knock instantly goes away at roughly 800rpms and higher. Its ONLY at idle, it doesnt get faster or louder as you give it throttle."

Then do not worry about. These engines are inherently noisy, and often way worse than yours and last forever anyway.

For what it's worth, I had the exact same symptoms surface after doing some work to my 88 4.0 a couple of years ago. I changed the lifters, did a bunch of stuff and generally panicked about it for awhile. I ran a bunch of different oils and filters and other things, but it stayed consistent. Finally I was forced to give up and learn to enjoy knocking.

Two years/many remote trips across the desert later, and the noise has all but dissapeared. :confused1: Jeep runs as fine as ever.

Royal Nonesuch said:

For what it's worth, I had the exact same symptoms surface after doing some work to my 88 4.0 a couple of years ago. I changed the lifters, did a bunch of stuff and generally panicked about it for awhile. I ran a bunch of different oils and filters and other things, but it stayed consistent. Finally I was forced to give up and learn to enjoy knocking.

Two years/many remote trips across the desert later, and the noise has all but dissapeared. :confused1: Jeep runs as fine as ever.

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I think mine is partly slop in the parts from 290,000 miles of wear on the pivots, rockers and rods in the tappet/lifter train. Plus worn valve guides that let the tappets load sideways on the valve stem sometimes. Maybe even old loose springs? But mine has gotten noticeably quieter lately, most of the time. :sunshine:

do you have the injector plugs hooked up to the correct cylinders? and does it run fine just with a little knocking noise? did you disconnect the battery when you did work? the renix ecu will save fuel trims and could take a bit to normal out otherwise.