I just found this article which talks about measuring pressure at WOT and states "Most exhaust systems are going to be in the upper 8-10psi range under full load. A decent aftermarket exhaust will peak out at around 4-5psi with some of the very large ones peaking around 1-2psi"

I'm hesitant to remove exhaust components to do more testing with because the whole system is welded together and it would be work to reinstall it. I'll use connections between components next time I do an exhaust.

I can repeat this test with the gauge in the downstream O2 port to see the pressure drop through the cat. That would be helpful info.

 

7.5 psi still seems high to me, but the values in the article seem to be based on actual measurements.

 

First thing that popped into my head watching that driving video was, "is his transmission and torque converter okay!?"

I haven't driven an AW4 Jeep in years but it seems like it really doesn't want to shift and/or lockup. Maybe it's just the video or how you were driving on purpose but it seemed "off" to me. Although, maybe I've been driving AX15 Jeeps for to long haha...

 

I was driving it oddly to purposely have it hang at higher RPM. Also was climbing a fairly large hill. The odd RPM surging while I was sitting still was likely because I had my O2 sensor disconnected. I do sometimes think my transmission doesn't want to lock up quite how I would expect it to, but its always seemed to be consistent and not an issue. Fluid is new and its had a cooler on it forever. Recently adjusted the TV cable.

 

Put a fuel pressure gauge on the rail to see what is going on with my fuel system because I thought it would be an easy thing to verify and rule out from being an issue. I wanted to rule the fuel system out just for peace of mind, and because I've never measured fuel pressure and want some baseline numbers. What I found just brought up more questions.

I did the 4 fuel delivery tests in the factory service manual, and the system passed them all.

1) Pressure should be 49.2 +/-5 psi at hot idle. I'm at 45.5-46 psi. In spec. Check, albeit lower than I'd like.

2) Pressure should not leak down past 30 psi in 5 minutes. I measured 42 psi at 5 minutes, 38 psi at 15 minutes. Check

3) Fuel pump should not draw more than 10 amps at full load. I measured ~3.5 A. Check. Turns out I actually did this one wrong. I had the fuel line disconnected and pumping into a container whereas I should've had the pump pushing pressure.

4) Fuel capacity should flow at least 250 mL in 7 seconds. I got about 300 mL. Check, although this one was a bit hand wavy. Also I'm just now reading the procedure says to remove the fuel filler cap, which I did not do.

So all checks out, right?

The part I'm unsure what to expect is the fuel pressure readings while the engine is operating under load. I ran the hose on the pressure gauge from the schrader on the fuel rail out the corner of the hood and taped the gauge to the windshield. Took it for a drive and wound it out to 4500 rpm while climbing a big hill while filming the gauge.

What I found was the fuel pressure dropped with rpm while under load. I saw it as low as 38 psi at 4500 rpm.

Is that normal?

Injectors expect constant pressure to flow properly and produce a properly atomized and shaped spray. If I'm only getting 38 psi at the rail, that's a problem. But am I actually getting that, or are my measurements inaccurate?

The fact that the FSM does not lay out a test under load like I did makes me think this is not a correct way of evaluating this. Maybe the hose was kinked too much where it was squeezed by the hood, or maybe the flow of fuel past the gauge is causing the readout to be different than what the injectors are seeing.

First video is close-up of the gauge:




Second I tried to get an angle that could see the gauge and the tach:





I replaced my pump with a Carter brand unit in 2023. No idea what the performance was when I installed it. I'm hesitant to jump right to the conclusion that I need another fuel pump, but its a possibility. I'm first going to rule out a pinched/kinked fuel line.


Unrelated: I measured the resistance in my spark plug wires to determine if I should replace them or not. I have some fancy-pants Magnecor KV85 wires that I installed 50-60k miles ago. The resistance values I measured ranged from 110-203 ohms/inch. The factory spec is 250-3000 ohms/inch. Hmm. Weird that my wires are lower resistance than spec. I would expect worn wires to have higher resistance. Quick google search showed the factory had a minimum spec on the wires to control EMI and electrical noise to the PCM. Also saw some talk about too low of resistance wires frying ignition coils. I also have an MSD coil installed, so really all the factory specs are out the window. They've worked fine for me for this long, I'm going to run them.

 

There are two approaches to fuel pressure control strategy. The most common approach for port injected vehicles is to control fuel pressure so that the pressure drop across the fuel injectors remains constant under all driving conditions. This means that as the manifold pressure changes (due to throttle position and engine load), the fuel pressure supplied to the injectors also changes. This is typically achieved by using a fuel (back) pressure regulator attached at or near the end of the fuel rail. A vacuum line from the manifold to the fpr is used to automatically adjust the fpr and the fuel pressure. This approach was used in early model XJs - I think the Renix models.

Later model XJs use a constant fuel pressure strategy. Prior to owning an XJ, I didn't even know this strategy existed. While its mechanically more simplistic than manifold vacuum referenced fuel pressure, I feel like it would be challenging to utilize this approach for accurate fuel delivery into the engine. In this approach manifold pressure is one of the inputs used by the engine control algorithm to adjust fuel injector pulse width. Seems to add a significant layer of complication to fuel injector pulse width determination in exchange for very little reduction in mechanical complexity.

Anyhow, bottom line is that the fuel pressure at the fuel rail should be constant no matter what the engine is doing. If its dropping down to 38 psi at high load, then your fuel pump is not getting the job done. While the reduced pressure you are seeing at high load is out of spec, the ECU can at least partially compensate for this via the short term fuel trim. However, the short term fuel trim is a reactive strategy and not particularly fast-acting on older ECUs, so the ECU has to see a lean afr before it will try to compensate, and that's only in closed loop. If the engine switches over to open loop, then the long term fuel trim will be used to adjust fuel flow, but it is unlikely to capture the lean condition that will occur due to short transients at high load.

 

I don't have much input on the fuel delivery. BUT, I do have a nugget in regards to the exhaust back pressure. I dragged my exhaust over a good sized rock ledge last spring. That crushed the cat in some. The honeycomb inside the cat broke and a big chunk of it wedged itself sideways in the inlet to the muffler. That was a joy to clean out. Caused the jeep to bog down, low on power and stunk pretty good. Don't think that's likely to be your issue, but worth considering if you've banged the cat up much.

 

Did a compression test yesterday. Did it engine warm, all plugs out, and throttle body wide open. I was pretty happy to see the results right off the bat:

Dry Test 1 / Dry Test 2 / Dry Test Average

1- 156, 154, 155
2- 152, 156, 154
3- 165, 171, 168
4- 164, 168, 166
5- 165, 168, 167
6- 149, 151, 150

Highest average was cylinder 3 at 168 psi, lowest average was cylinder 6 at 150 psi. 18 psi / 11% difference between high and low. Not bad.

Then I did a wet test. Added 1 tablespoon of oil to each cylinder. These results were, less good:

1- 199
2- 186
3- 209
4- 193
5- 203
6- 198

That shows an average of 38 psi increase across all cylinders between wet and dry. While I don't exactly trust the numbers I am seeing because they all seem quite high to me, the trend in the data that wet numbers are significantly higher than dry makes me think my rings/cylinders are worn. Good news is that everything is pretty consistent between cylinders. Nothing is really standing out to me as being a problem, but this motor is likely tired.

I'm no expert in how to interpret this data. This is the first time I've ever done a compression test. I want to do some more googling about interpreting these numbers. I'd be curious to hear other people's thoughts on the results.

The factory spec is 120-150 psi, but states the engine should only be rotated 3 times before reading the result. I was cranking 6 revolutions to try to get the highest reading possible. I didn't read the procedure in the manual until after I was done, so kinda screwed myself there.

 

How many miles on your engine?

Since you are really getting into all this, (I'm really enjoying your posts!), you could also do a leak down test to help determine the "health" of your engine.

 

Thanks for commenting and adding your 2 cents! Its nice to know other people are keeping an eye on this. I've learned a ton from reading other people's build threads over the years. Its a nice place to be able to dump my thoughts and some data for myself to reference in the future, but also a great resource for everyone. Half the reason I'm going through these tests is just to learn how to and get a baseline for what "normal" numbers are.

My Jeep just rolled over 200k miles. I may do a leak down, but not really feeling like I need to right now. The big jump from dry to wet compression test has me thinking my rings are worn. My next steps are to do a steam clean and Mopar Combustion Chamber Cleaner treatment, as well as distributor cap, rotor, plugs, throttle body/IAC cleaning, fuel system cleaner, and refresh the CCV system. I also am going to add in a can of Restore into my oil. I'll probably do another compression test after this to see if there was any change.

I also just ordered a fuel fitting to do a fuel pressure test at a different point in the system. I'm curious to see if I can reproduce the pressure drop by testing in a different spot. If the results repeat, I'll be doing a fuel pump as well.

 

I used to run a can restore in an old worn out Ford Ranger I had. It was 300k+ and wouldn't climb a hill without down shifting. A can of restore and it went up the same hill much easier. My butt dyno say it definitely helped with compression.

Have you looked into the Valvoline Restore & Protect? It will clean up the rings (and everything else) and many people claim it has solve oil burning issues. Evidently it removes all the deposits and carbon that's burned onto everything to the point it can potentially overwhelm the oil filters ability to capture it and keep the oil flowing. Folks report having to change the filter out half way through the oil change interval. (kinda scary lol) There are a ton of you tube videos on it, some more credible than others.

 

That's the first I've seen that new Valvoline oil. Looks pretty sweet and is getting good reviews. Too bad its not available in a 10w-30. I used to run 5w-30, but have since switched back to 10w because I typically don't run my rig in the winter. 10w-30 is shown as the preferred viscosity for the climate I use my rig in.

These two graphics are from a 1997 and 2000 factory service manual. The 1997 shows 5w-30 being acceptable up to 60°F, but they actually reduced that down to 32°F for the 2000 model year manual.

1997:






2000:

 

That's an interesting interpretation of those graphs. Oil viscosity ratings are a rabbit hole for sure. I was of the impression that at operating temperatures, a 5w30 and a 10w30 have the same characteristics.

For a quick explanation the amsoil blog has a good short article on it.

https://blog.amsoil.com/5w-20-vs-5w-...he-difference/

 

Its correct to say that a 5w-30 and 10w-30 have the same characteristics at operating temperature. Where they differ is on cold startup, which is arguably the most important time for an oil to perform. The graphs in the FSM's indicate that a 5w is actually too thin in higher temperatures. This can result in the oil shearing and losing its protective properties prematurely.

This all needs to be taken in the context that the manuals were written 25+ years ago at a time when oil technology was not nearly as advanced as it is today. The oil viscosity debate will likely rage on forever. I'm just gonna stick with what the manual says.

 

Still been puttering around with this thing a bit. I cleaned the engine by pouring water down the throttle body. Dumped probably half a gallon into it, then followed that with a can of Mopar Combustion Chamber Cleaner. Beat the crap out of it for a good 15 minutes or so after. I didn't get quite the smoke show that people describe when using seafoam, but I got some. No noticeable difference in performance, but I feel better knowing that's probably the first time its been done in 200k miles.





After that I replaced the CCV valve, valve cover gasket, distributor, cap, and spark plugs. I used a Mopar brand CCV valve, a made in USA all brass distributor., and NGK spark plugs.

I did not replace my plug wires or ignition coil because in 2015 I bought some fancy Magnecor wires and an MSD ignition coil. All of the plug wires ohm'd out to be good, BUT, when removing them I damaged one by pulling the terminal out of the boot. I called Magnecor and they sent me a new one, no questions asked. Talk about good customer service!








Got all that buttoned up with some fresh Mobil 1 oil, a can of Restore (brand) engine restorer, and a Mopar oil filter.

Again, no real change in performance, but it still starts right up, idles smooth, and seems to run good at lower RPM. I still can't accelerate up my hill though.

I checked my fuel pressure again in a different spot. I set this gauge up by the connection under the driver's footwell area, and rigged up this ridiculous setup to film the gauge while I drove. It produced a video that would make bigfoot videos look like 4k, but I was able to verify the same result that I saw when measuring at the rail; my fuel pressure is dropping at higher RPM. This reinforces that my next move is to do a fuel pump.





Part of my learning that I've been going through has been centered around engine tuning and how I've bolted a bunch of parts to this thing without really knowing their affects. Namely, the injectors and throttle body.

I have 784 injectors which flow about 7% more than stock at nominal fuel pressure. This would make my rig run rich in open loop when the PCM cannot trim fuel out. The lower than nominal fuel pressure might actually be helping me somewhat with keeping my AFR closer to stock. I also have a 62 mm throttle body which could potentially affect the MAP sensor readings. Math says this should be less than 1%, but worth mentioning as its something that I spent some time thinking about. This in theory would make it run lean and effect the behavior of the TCM, but its likely negligible. I'm considering going back to the OEM size throttle body to get some throttle control back, and it seems like the OEM throttle body is more than capable of flowing enough air for a stock 4.0.

Last, but not least, a treat especial! I had no intention of buying one of these, but when one came up for sale 10 minutes up the road from someone who didn't know what it was, I had to have it:





That is a NP241J transfer case out of a 2006 TJ Rubicon. That's right, 4:1 ratio. I paid $500 for this, which is an absolute steal for what they go for. I actually offered him double his asking price because he said someone else was already interested. Ran up the street and had this in my hands. Now THIS is an upgrade. I'm sticking it in the corner for now, but it will match up very nicely to the built JK axles. Not only are these 4:1, but they're also quite a bit stronger than the 231's. 4:1 with 5.13's is gonna be stupid low. I can't wait.