Ah, that's the old style TPS. The '06+ have a very different throttle body, and these were far less prone to the Restricted Performance errors.

Gut feeling, I'm not leaning toward the TPS anyway. That wouldn't be limited to only uphill stumbles, I should feel that more often. Whereas my issue only occurs under light loads. My instinct is pushing me toward the O2 sensors, since it seems that the PCM is learning something incorrectly (going back to when I could reset the battery and the issue would clear up for a few days). However, the fact that both my O2 sensors indicate a sudden rich spike during the stumble has me looking at fuel pressure controls instead. Unless it's just learning some bad fuel block. I mean at this age/mileage the O2s couldn't hurt as a first guess. The fuel rail pressure sensor is $100 on a whim.

I think I need to get SDD working on my new laptop so I can drive around and see better data.

 

Unfortunately, at some point, changing parts one by one is the only way left to solve issues. It can get expensive real quick though.

 

You can test that by pulling F33 & 34 in the engine bay
This is the O2 sensors. The ECU will stay in open loop and run without the sensors

You will generate some codes for the sensors but ignore and clear them.

Cheers
34by151

 

Fuel rail pressure sensor - not common from my experience, but have seen a few over the years go faulty. Do you get a code P0087?


These sensors are the same for the 4.2L N/A, 4.2L S/C, whether its XJ, XK, XF or even S-Type. If you decide or need to replace this, check pricing. One model might be more expensive than the other even though its the same part.


Also check the vacuum pipe for this sensor as well, they are plastic and do deteriorate over time.

 

No codes at all. Yesterday I was watching the FRP sensor in graph mode, and whenever the stumble occurs the sensor is dropping down to 15-20 psi. Once the stumbling stops, the FRP stays between 40-50 psi. If I had an adapter set I'd put a gauge on the rail and see what it was doing to confirm.

Have already checked the vac lines going to the FRP sensor, they are all fine.

Before I throw a new sensor on it, the other idea I'm tossing around as a possibility is a fuel injector issue. A single sticking injector would definitely cause one bank to go rich, but could it cause both banks to run rich? That would be the only other scenario I can imagine causing both front O2s to peg rich and fuel rail pressure to drop. However, I don't see how a single injector stuck open could cause the drastic drop in FRP the way I'm seeing it in my datastream. Generally you'd need an injector stuck on for several seconds to drain the rail pressure, not a sub-1 second timeframe.

 

You'd expect codes for a stuck injector (AND the PCM would almost certainly try to disable it). So, doesn't sound like it's a stuck one.

 

I'd agree with JagV8 - I've seen quite a few injectors fail on the 4.2's, and its either logged an actual injector code or caused a misfire code for that cylinder, or its in PCM itself.


The fact the FRP is showing a drop in pressure when the stumble happens suggests to me that its fuel related. Does the local Autozone or O'Riellys have a fuel pressure gauge kit you can rent? You really need to watch it with a gauge to see if that is reflecting the same drop in pressure as the FRP, if so, pump may be your issue....

 

if you cant put a gauge on the fuel rail the next best option is a cheep vac gauge.

Assuming you see a vac drop when it stumbles it wont be the fuel pressure. What you will be seeing with the sensor reading is the relative pressure. That is relative to the intake air pressure.

Possible sources for this are electrical, intake or exhaust leaks.
On the intake side Id bee looking at the valves and solenoids such as the evap, egr ect

If you had a permanent leak you would not have an intermittent issue. The above are ecu controlled and if one is faulty may cause a stumble when enabled by the ecu and may explain the lack of codes.

Best advice I can give is cover the basics first. Update the earths, check air with a vac/boost gauge, fuel with a pressure gauge. Once you have that start looking at what is commanded by the ecu when you get the stumble.

Lets say that is the egr, run the car with it unplugged and ignore the codes, see if you remove the problem and replace that part. Randomly swapping parts is going to get expensive real quick

Cheers
34by151

 

Agreed--I do not see this as a permanently stuck injector. Possibly an intermitently stuck injector, but that would have no relation to whether I was going uphill or not.

An adapter kit like this one?
Also agree completely. However, once I unplug the EGR or O2 sensors (or whatever is being tested), won't the car default to speed-density mode with a CEL on? That might end up clouding the issue because I'm sure it will run very differently with the base mapping.

I will check some more things tonight. I wish it was as easy as putting on a fuel gauge. If I could replicate the stumble reliably I would do that today. But as it is, the issue is too intermittent. Also working on getting SDD up to view additional datastream items.

 

if you remove the o2 heater fuses the ecu will generate some codes but stay in open loop

Under normal conditions the engine starts in open loop then moves to closed loop after warmup. To keep it simple open loop uses the stored map in the ecu for fueling when it switches to closed loop it follows the the 02 sensors using the fuel trims

In the case of the O2's you will get heater codes and a an orange mil but ignore them it is not causing any harm. Same goes for the evap and egr just codes relating the the part as failed but not harm will result.

Cheers
34by151

 

JBzXJ40, does this tenfold MAF increase remain true when the car is static and revved? I would imagine that a good part of the increase would be as a direct result of the cars forward movement causing a 'Ram Air' effect? I ask, because as my attached pics show (with the car stationary) my MAF only increased by as much again from idle?
Regards,
Ray

 

Ray, you will see differences with the supercharged as compared to the NA engine. But the MAF reading change with RPM. The "ram Effect" is not the major factor.

It would be better to start a new thread so we can help you as best we can with your problem rather than confuse the issue Michael (mhamilton) is having. The more detail you can give the easier it will be to help

Cheers
34by151

 

I may owe you a beer!

Last night I got a chance to check for vacuum leaks with my MitiVac... I definitely found *a* problem (not sure yet if this was *the* problem). The AIR switching solenoid is bad, leaks on the manifold vacuum side. This is sharing the same 3/32 vacuum hose as the fuel pressure sensor.

I pulled the AIR solenoid and capped the line. The car definitely runs differently now. Have to wait for it to adjust to this new condition before I will know if my stumble has returned. Can definitely see why this would affect the FPR--a leak in the same line would reduce the vacuum signal to the FPR.

Also found after letting the AIR solenoid sit inside overnight, it leaks less. Will get a new one ordered today.

The one part I've not been able to find is the little rubber boot at the end of the nylon vac hose. Mine is pretty dried out from age. Anyone know if I can order those separately?

 

From the part number on the solenoid that is the secondary air injection solenoid

This should have 2 hoses
The 3 way one is C2C22717 (part 6)
The one you pictured is XR849471 (part 3) and should connect to the diverter valve




Cheers
34by151

 

Yhatzee!

 

Well, nuts. The stumble is still there, along with the fuel pressure glitch.

On the upside, my fuel trims have never been this good! I noticed the fuel pressure is about 5 psi overall higher than before, and my LTFT have dropped to 0% and 0.7% (around 6% before).

But still the same scenario, cruising on the highway with fuel pressure at 50-55 psi, depress the throttle slightly for an incline and bang, the FRP drops to 10 psi for a second, then back to normal. Did this a couple times on my drive home. Also I sat at a long stoplight and noticed one glitch even while idling.

Working on getting SDD setup. Will post back when I have more data to share.

 

Good news on the trims

Do a test with the car locked into open loop by removing the O2 heater fuses.
This will stop the ECU opening some of the vac lines such as the evap system.

Cheers
34by151

 

Will do this later in the week. I did a static check on the evap purge solenoid to make sure it was fully closed to vacuum (and it is) . But I think it's a PWM controlled valve and I guess it's possible it could be getting sticky. Will be easy enough to check.

Stay tuned...

 

It's been about 2 weeks and 150 miles since I fixed that vacuum leak at the air solenoid. The car is running much better overall. The PCM was definitely re-learning as I've noticed the shifting has improved.

For the most part the stumble is gone. Driving you would never notice any issues if you didn't know to look for them--and I'm fine with this. However I think there is still an underlying issue, it's just not as noticable anymore with the engine running so much better.

Here's what happens: when I watch my scan data for the tell-tale fuel pressure dropout it almost never happens anymore. I say almost, because yesterday I did see two blips (the FRP dips to 10 psi for a split second just as before) that happened in quick succession, though they are not causing the harsh engine stumble they did prior to fixing the vac leak. Normally I see the fuel pressure remain between 42 and 60 psi per the rail pressure gauge. When I catch the dropouts there's just a very slight feeling of a miss.

But the frequency of these blips happening is far less than before fixing the leak. Previously it would stumble every time I went up a slight incline. As I mentioned, yesterday on a 15 mile drive it only bliped twice and that was it for that drive. Not sure I can see blaming the FRP sensor anymore. Definitely do not think it is electrical (as in a bad wire or connection), as that would not have been reduced in frequency by fixing a vacuum leak.

Side note, my fuel trims at idle are nearly zero which is great. But cruising on the highway they're +8%. Before the fix they were only +5%. That may be a red herring though.

 

If I'm understanding you correctly the drop in the fuel pressure sensor reading only occurs with the "blip"

If that's correct it is a not related to your issue as the reading is relative to vac. If the engine "blips" you will get a vac change causing the reading. I very much doubt you would see any change with an actual fuel pressure gauge on the rail. Ie an absolute reading rather than relative.

Id go back to the earth points next. Upgrade G1,G2,G3. These are the headlamp ones. Even if these dont change the issue your avoiding other issues that will occur down the track. Drill a hole next to the old earth and use a stainless M8 bolt. Shift the lugs over to the new one and add a cable to the old lug. Its easy if you take the bumper off. For G2/G3 you can add the new one between the old ones. For both sides you can pass the bolt in from behind the headlamp

While on earths take a look at G26 (LHD) / G30 (RHD)
This is the earth for the ECU, Coils & wiper motor. After cleaning up the lugs add a new cable and run it to the headlamp earth.

Cheers
34by151