I am working on a problem with a '94 XJ40.

The car runs great, well, it did.

Driving through Phoenix yesterday (ambient air temp above the road was about 125F) the car stalled. VCM indicated a code 26.

Symptoms:
- Engine hesitates/bogs/misses under any throttle
- idle is smooth at start then gets rough
- idle air hose seem to have been popped off due to a misfire in the intake

All of this only happens when the air is hot. Back home here in the SE mountains it is cooler and the car runs fine.

It has not thrown a code 16 (air temp sensor) but I am leaning that way thinking maybe the sensor is ok in low-mid temp ranges.

I did pull the elbow off the intake today and clean the sensor. Also pulled the fuel filter off to check it for clogged. Air passes easily although some compressed air applied to the outlet blew a lot of silt out of the filter. I'll replace it tomorrow just for grins.

Also checked the gas cap to verify it was tight. It is OK.

After cleaning the filter and sensor (and all connectors) the engine starts right up and runs fine. VCM gives no codes (although I do need to clear the CEL).

Any thoughts? Comments?

I suppose the crank sensor is another possibility but only when hot?

-Mike O

 

Hi Mike,

On a '94, Fuel Fail 26 indicates that the Oxygen Sensor signal is reporting a lean exhaust condition that may be due to an intake air leak or vacuum leak that is allowing unmetered air into the system. The idle air hose you mention that became disconnected may have triggered this code. Is this the hose that connects to the Idle Air Control Valve? On our '93 that hose is fixed to the IACV with a hose clamp - if your hose isn't it might be worth adding a clamp.

From your post, I am uncertain as to whether or not your VCM showed FF16 as well. Could you please clarify?

Regarding the rough running after initial startup, that could indicate a problem with the O2 sensor. At startup the Engine Control Module (ECM) ignores the O2 sensor until the sensor achieves operating temperature of 500C or more, which takes around 20 seconds or so. Once the O2 sensor is up to temp, the ECM can go into closed-loop fuel metering. It may be that your rough running begins when the ECM goes into closed-loop. You can check the O2 sensor with a high-impedance Volt-Ohm Meter (VOM). The voltage should swing between just greater than 0V to just under 5V. If the signal seems lazy or is fixed at one voltage, suspect O2 sensor failure.

Other sensors that provide primary inputs for fuel metering are the Intake Air Temperature Sensor you cleaned, the Coolant Temperature Sensor, and the Mass Air Flow Sensor (MAFS). You can check the outputs of the IATS and CTS with an ohmmeter to be sure they are in spec for a given ambient air / coolant temperature respectively. If the signal from either of those sensors is faulty, the ECM may incorrectly enlean or enrich the air-fuel ratio (AFR). The MAFS is more difficult to test on an XJ40 but you can clean the hot wire by flushing it with zero-residue MAFS or Electronic Cleaner spray, and clean both halves of the electrical connector as well.

Have you checked to see if there are any other codes lurking behind the FF26? You can clear codes one at a time to see if any are hiding behind the one displayed by turning the key to Position II (AUX) without starting the engine, then removing the Instrument Pack fuse in the center fuse box under the arm rest (on our '93 it's Fuse 9, the second fuse from the left). Watch for the Speedo and Tach needles to drop below 0, then wait about 10 seconds and reinsert the fuse. Now turn the key to the OFF position and wait a few seconds. Then turn the key back to Position II (AUX) and press the VCM button to see if there is another fault code displayed. If so, make a note of it, then repeat the process until no codes are displayed.

The misfire and hesitation you mentioned could also be related to an intake air leak and lean running condition. It might be worth misting some starter fluid around all the vacuum line fittings, intake air plumbing joints after the MAFS, throttle body, intake manifold, breather hoses, etc. to see if the engine rpm increases when you mist starter fluid around a specific area.

Driving in extreme heat as you do can certainly cause odd behavior, so it may be that one or more of your sensors is "on the edge" of its original specification and breathing hot, rarified air may just push things over the edge.

Let us know what you find.

Cheers,

Don

 

Thank you Don.

As luck would have it the Haynes manual I purchased has arrived. The manual seems to have all the procedures and values for checking each of the points you recommended. Now to sort through them. As an aside I use an unlit torch with propane or MAPP to find vacuum leaks.

The 16 was something I induced. I pulled the connected free while doing a quick check roadside. The 26 can be reproduced by pulling the IAC host free.

I was unaware of how to cycle through the other codes. The only code I saw roadside was 26. Now I'll have a closer look.

About the MAFS: I noted that there does not seem to be a wire spanning the throat but the does appear to be a port in one side of the sensor. Is this the normal configuration for this sensor?

Regards,

 

Mike,

Based on the info in your first thread, my thoughts gravitated towards the o2 sensor as well. I had the same symptoms and replaced mine several years ago.

 

I should have given credit to Bryan N of the Jag-Lovers XJ40 forum, who taught me that method of checking for multiple codes.

Thanks Bryan!

Don

 

Yes, that's the normal configuration inside the XJ40 MAFS. I think the wire is in a channel where you can't see it, but you can carefully flush everything inside the throat with MAFS cleaner spray, getting your spray tip as close to the channel as possible without touching anything inside the channel with the tip. Clean both halves of the electrical connetor also, since even slight additional resistance from oxidation or contamination can affect the signal received by the ECM.

Cheers,

Don

 

OK, so no other codes. The 26 was caused by the idle air hose. The 16 was caused by me (pulled the plug on the sensor).

I was reading in the Haynes manual and noted a comment about earlier cars. Apparently there was a hot running issue and the dealer fix was dual fuel pumps?

I think it may be worth the time to check the FPR and related pressures first. If the fuel was boiling due to low pressure and high heat then it could very well explain the problems.

 

Rob,

In collecting intel about the problem my wife, who was driving at the time, noted that the engine coughed/sputtered/popped (in that order) before the idle went bozo then the engine died.

I am assuming it was a backfire in the intake that popped the idle air hose off. That started the spiral. There were no other codes.

I am wondering if there is enough possible advance in the ignition that it could have backfired through the intake.

The FPR is low-hanging fruit as is the O2 sensor. I may change out both if only because I have no info on the service history of the car.

Regards,

 

Hi injuhneer,

Around here we generally discourage "throwing parts at a problem," but since FPRs don't last forever and it's easier to replace one than it is to test it, it would be a good idea to just do it. I found a wide range of prices, as low as $40 on eBay for brands like Beck-Arnley, but I opted to go for an OE Bosch for a little less than $80 via an Amazon seller. The Beck-Arnley probably would have been fine. The photo album at the link below shows the process of replacing the FPR:

Welcome to Jag-lovers - Members Photo Viewing Page


The other component that works in conjunction with the FPR to maintain pressure in the fuel rail is the Check Valve or Non-Return Valve. There are actually two, both built into plastic components in the fuel tank. One is just ahead of the fuel pump, to keep the pump primed, and the other is just after the output of the pump, to keep fuel pressure from escaping back into the tank when the pump stops. The plastic canister and evaporative flange assemblies are no longer available, so it's easiest to just add a new check valve in the fuel line. Below are photos showing how I added one to our '93 using an inexpensive valve from McMaster-Carr:

Welcome to Jag-lovers - Members Photo Viewing Page


The O2 sensor also doesn't last forever, so it's not a bad idea to replace it preemptively, but since it's easy to test if you have a decent VOM it might be worth doing so just to know if you are actually addressing the problem. There are a number of quality options for replacement O2 sensors, but I again went for an OE Bosch. Below are links to photo albums that show the process of replacing the O2 sensor on our '93, which I believe is identical to your '94:

Welcome to Jag-lovers - Members Photo Viewing Page
Welcome to Jag-lovers - Members Photo Viewing Page


Just to be certain you get the right part number, you can search your model and year at the Bosch Parts Finder:

Bosch Vehicle Part Finder


Cheers,

Don