04 4.0 reduced power, popping through intake - JeepForum.com
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post #1 of 28 Old 03-07-2020, 04:50 AM Thread Starter
Rixdafix
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Solved: 04 4.0 reduced power, popping through intake

Long story short, it appears to have been the Park/Neutral switch. Read on for symptoms or if you're just bored.

Alright so I had to replace my rusted out fuel tank brush guard anyway (with a skid plate and with 175,000 miles and some long road trips planned, there was no way I was going to not replace the fuel pump and filter/regulator while I was back there. Also, I already replaced the crankshaft position sensor as a precaution. All parts are OEM.

The problem I experience started as a very occasional hiccup where it would lose power for only a moment and clear right up. It would also backfire a little through the intake. The problem is now constant which is good because it will make diagnosing easier. Right? RIGHT? There are no codes and no CEL. I use the torque app and a PLX Kiwi for reading codes and data.

So here are some observations.
The problem is present cold or warm, no difference.
The little bit of data I can read looks normal, O2, throttle position, etc..
This thing will idle for days without missing a lick.
If I very lightly accelerate, it still runs poorly but I can get it up to 40 or 50 MPH on flat ground, forget going up a hill.
If I try to accelerate normally, it cuts in and out (but not a misfire) with much reduced power and light intake popping/backfire.
It never dies and never has a dead miss, all cylinders keep firing, it just runs weak.
Even when I'm coasting, no throttle input, I can feel it bogging down a little by way of engine braking if that makes sense.
It also has an extended crank of a couple of seconds before it starts.
What it feels like is someone pinching a fuel line partially closed or retarding the timing.
Being an 04 it does not have a schrader valve on the fuel rail but I have one I might swap in so I can test fuel pressure.

In addition to repairs mentioned, I have cleaned all under hood grounds and battery connections and replaced almost every vacuum line, all as precautions. I also re-seated the PCM connectors and checked the harness above the valve cover.

The day before I parked it to do the fuel tank, I washed the Jeep including the undercarriage at work. I did NOT wash under the hood and it was simply garden hose pressure, no power washer. About 10 minutes into the drive home, the reduced power problem started again but it is continuous now and has not run properly since. I hate to even mention that because I think it's coincidence but yeah there's that.

Again, I have replaced the crank sensor (Mopar), fuel pump (Bosch), and filter/regulator (Mopar).

I'll probably swap fuel rails today so I can get a pressure reading but I'm open to suggestions.

I'm leaning towards the camshaft position sensor.

What say ye and thanks in advance for any help.

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post #2 of 28 Old 03-07-2020, 05:56 AM
jtec
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Before going through trouble of swapping fuel rails - with your scanner look at
Pending codes- we might be lucky and see something.

The live data- ECT, o2 sensor voltages precat sensors, MAP, fuel trims long and short.

Try old school vacuum leak check using carb cleaner.

PS is your AIRBAG light on?

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #3 of 28 Old 03-07-2020, 06:21 AM Thread Starter
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Quote:
Originally Posted by jtec View Post
Before going through trouble of swapping fuel rails - with your scanner look at
Pending codes- we might be lucky and see something.

The live data- ECT, o2 sensor voltages precat sensors, MAP, fuel trims long and short.

Try old school vacuum leak check using carb cleaner.

PS is your AIRBAG light on?
No codes, pending or active. I tried to drive it long enough to throw a code but it was running really crappy so I parked it.
I "think" the live data looks good but if someone could clarify min/max O2 volts and trims that I SHOULD typically see that would be awesome.
Pretty sure there are no vacuum leaks as I went over the whole system and it idles perfect but I will still double check.
No airbag light on but it is functional.
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post #4 of 28 Old 03-07-2020, 06:43 AM
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Easier if you post data.....post the o2 voltages and fuel trims a simple+or - number.


Airbag light ON but functional? explain please.

Does cruise control and horn work...?

Thinking is the Airbag problem can affect engine sensors (TPS) and create drivability issues.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #5 of 28 Old 03-07-2020, 06:54 AM Thread Starter
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Quote:
Originally Posted by jtec View Post
Easier if you post data.....post the o2 voltages and fuel trims a simple+or - number.


Airbag light ON but functional? explain please.

Does cruise control and horn work...?

Thinking is the Airbag problem can affect engine sensors (TPS) and create drivability issues.
OK I'll record another road test later today and post some numbers.
You misread, NO airbag light on. Illuminates momentarily on startup.
Cruise and horn do indeed work.
Thanks for the replies by the way.
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post #6 of 28 Old 03-07-2020, 01:32 PM Thread Starter
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So I rechecked some wires in the harness and the connectors at the PCM. This thing is really random as to how severe it acts up and how often too. This morning I could hardly get to 40 mph but just a little while ago it ran fine up to about 60 mph. I had the PCM out so I could clean out some leaves and pine needles and the photo shows what the back of the PCM looks like. I wonder if this is a concern. The rust doesn't appear to go through but yeah.

Here's a track recorder video I made on the Torque app it acting up.


A little over half way through the video I go half throttle and it acts up at 62 mph and again at 70. You'll see the tach briefly drop a few hundred RPM when it happens. I do lift the throttle for a second right after the first episode just in case you see it and think it's a TPS glitch.

It acts just like the fuel is shutting off but it never actually dies. I think I'm gonna throw that other fuel rail on this afternoon so I can get a gauge on this thing.
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post #7 of 28 Old 03-07-2020, 02:34 PM
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CPS failing?
My reasoning is that popping through the intake indicates ignition timing and the CPS controls that.
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post #8 of 28 Old 03-07-2020, 02:35 PM
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check fuel pressure and do a fuel pressure drop with KOEO.

easiest is attach gauge, turn key to RUN - observe high pressure @3 seconds, leave key in RUN, post pressure after 5min.

What is ECT?

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #9 of 28 Old 03-07-2020, 03:12 PM Thread Starter
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Quote:
Originally Posted by wjkeith View Post
CPS failing?
My reasoning is that popping through the intake indicates ignition timing and the CPS controls that.
Brand new Mopar crank sensor about 3 months ago as a precaution only.

Quote:
Originally Posted by jtec View Post
check fuel pressure and do a fuel pressure drop with KOEO.

easiest is attach gauge, turn key to RUN - observe high pressure @3 seconds, leave key in RUN, post pressure after 5min.

What is ECT?
Yeah getting ready to head to the parts store for some injector o-rings so I can swap that rail. I'll get a reading tonight or tomorrow.
ECT is right around 195-200 range.
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post #10 of 28 Old 03-07-2020, 06:44 PM
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How old are the O2 sensors? What do the wires look like for the O2 sensors. Are any of them laying against the exhaust? I had a horrible experience with my XJ, and I have been waiting for it to happen in the WJ....Maybe I'm just snakebit. Don't know likely someone else knows better, but the 0.9v on the O2 sensors seems a little high. I fear it is running poorly because the air/fuel mixture is off.

Matt 7:7-11
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post #11 of 28 Old 03-07-2020, 06:45 PM
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What about a plugged up catalytic converter? It would also make it cough and weez through the intake if the engine cant get the exhaust out. Just undo the exhaust at the manifolds at run it, see if that makes any difference. Just dont do it at 10pm at night unless you hate your neighbors....

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post #12 of 28 Old 03-08-2020, 08:30 AM
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ECT seams LOW. should be @220F.


Exhaust restrictions-
We can look at MAP readings or use a direct attachment vacuum gauge or a simple exhaust back pressure gauge. Again both vehicles at same time....

PS the data needed is at idle and @150 0rpm steady, the snap throttle is confusing.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #13 of 28 Old 03-08-2020, 08:59 AM Thread Starter
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I sure appreciate everyone's suggestions and help. Swapped fuel rails last night and read pressure this morning.
Results: KOEO fuel pressure 50 PSI within 3 seconds, turn key off pressure only dropped about 3 PSI over 5 minutes.
Of course it drove perfect for first 20 minutes or so then blam out of the blue started acting up. Falling on it's face.
Fuel pressure never fluctuated or dropped below 50 PSI. I think I can rule that out.

Quote:
Originally Posted by retrvrtrainer View Post
How old are the O2 sensors? What do the wires look like for the O2 sensors. Are any of them laying against the exhaust? I had a horrible experience with my XJ, and I have been waiting for it to happen in the WJ....Maybe I'm just snakebit. Don't know likely someone else knows better, but the 0.9v on the O2 sensors seems a little high. I fear it is running poorly because the air/fuel mixture is off.
O2 sensors appear original, I have no history of repairs from previous owner. Wires all look good and routed properly, just checked all four last night. I'll have a closer look at voltages.

Quote:
Originally Posted by Bigrigr View Post
What about a plugged up catalytic converter? It would also make it cough and weez through the intake if the engine cant get the exhaust out. Just undo the exhaust at the manifolds at run it, see if that makes any difference. Just dont do it at 10pm at night unless you hate your neighbors....
Can't rule it out or maybe something internal to the muffler like a baffle folding over and blocking outlet maybe.

Quote:
Originally Posted by jtec View Post
ECT seams LOW. should be @220F.


Exhaust restrictions-
We can look at MAP readings or use a direct attachment vacuum gauge or a simple exhaust back pressure gauge. Again both vehicles at same time....

PS the data needed is at idle and @150 0rpm steady, the snap throttle is confusing.
I'll recheck ECT.
I'll see if I can pull up MAP on Torque app, regardless, I'll also hook up a vacuum gauge to verify.
Didn't understand "Again both vehicles at same time..."
So the fuel trim and O2 I recorded earlier on road trip should actually be just recorded while parked and hold idle at 1500?
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post #14 of 28 Old 03-08-2020, 09:20 AM
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why did you turn key OFF?
You leave key in RUN position with eng OFF.
We want injectrs energized to test for issues.

As mentioned .9 volts on o2 sensor is HIGH and the fuel trims are borderline, we want to see single digit adjustments. The engine being cold MAY be a factor - we need to look at what we see as wrong. But all that revving engine makes it hard (for me anyway) to read the gauges in video.

You have done nothing to the jeeps lately, new parts, seafoam, filters, oil, plugs, I am leaning toward a exhaust back pressure test... just a hunch

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #15 of 28 Old 03-08-2020, 10:13 AM Thread Starter
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Quote:
Originally Posted by jtec View Post
why did you turn key OFF?
You leave key in RUN position with eng OFF.
We want injectrs energized to test for issues.

As mentioned .9 volts on o2 sensor is HIGH and the fuel trims are borderline, we want to see single digit adjustments. The engine being cold MAY be a factor - we need to look at what we see as wrong. But all that revving engine makes it hard (for me anyway) to read the gauges in video.

You have done nothing to the jeeps lately, new parts, seafoam, filters, oil, plugs, I am leaning toward a exhaust back pressure test... just a hunch
Oh my bad on the fuel test I turned key off as if I were checking for a leaking check valve. So key on engine off energizes injectors? I figured they were only triggered when running.
I can redo that test if needed.
Engine coolant temp and MAP added to screen, it's up to proper temp.
I have experienced this loss of power with engine still cool and up to operating temp.
For the record, I have not experienced the extended crank time since I replaced the pump and filter. Only current problem is the random power loss.
Only reason I recorded previous test drive and revved was to get it to actually malfunction and try to record as it was happening.
Have replaced pump, filter, crank sensor, all OEM new parts.
Made two videos. Couldn't hold 1500 steady, closer to 1600 in first video. Second video is at idle just because O2 1x2 wasn't fluctuating AT ALL and I wanted to show that.
Also took it on another test drive with all levels of throttle input and of course it didn't act up once, ran great.
It's a beautiful day, think I'm gonna put the wrenches down for a while, grab the wife, jump in the XJ, and go for a hike somewhere.


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