98 XJ8 77000 miles.
Original secondary tensioners (it's on my list to do).
Bought the car with upper O2 sensor codes showing. Replaced both.
New NGK Iridium plugs.
Replaced intake duct to fix air leak.
New air filter.
Cleaned throttle body, actually it was clean from previous maintenance.
Codes gone, running great, but no heat.
Fixed heater pump, replaced heater valve no heat.
Replaced the octopus and found a bad one-way valve.
Replaced thermostat and tower.
Heat finally !!!

Here's the problem, now that the the engine runs up to temp, I have a misfire under load. It runs fine when cold, then gets worse as it warms up.
When it's cold, it goes like a striped back gorilla. Hot, not so much.

There are no codes showing since replacing the O2 sensors.

Replaced intake manifold gaskets, part load breather hose. No obvious leaks, but I doubt that could be the problem, since it gets worse under load and at WOT. I went all over the engine with propane, no change in O2 readings or STFT. Random miss at idle. Sometimes it idles fine.

TPS shows smooth change from closed to WOT, and butterfly moves smoothly.

I have a spreadsheet starting from cold, but it ends quickly when my laptop battery quit. However, it shows the LTFT from the last run getting set back to low values, then about the time the battery quit, it goes high on bank 2. The problem seems to be bank 2.

Since it has never set a misfire code, I doubt that it is a coil. No oil around the plugs.

Is it possible that fuel injectors could do this? Wouldn't it be worse when cold?

Fuel pressure remains around 45-50 at idle or under load. Replaced the fuel filter just for good measure.

I have attached the cold start log and a hot log and would appreciate any help.

Bill

 

The only thing I can see on your cold start log is that the short term fuel trim stays at zero until the engine reaches about 70F. On mine it starts working from startup. After that the numbers seem to be OK. Have you cleaned the MAF sensor?
Check connections while in there. Maybe check connector on temperature sender.
Check connector block for O2 sensors, behind motor on firewall.

 

Thanks for the suggestions.

I have cleaned the MAF multiple times and checked the connectors.
I ran an additional test where I logged TPS, MAF, RPM and Load. Based on that and what I have read, I believe that the MAF is bad.

What stood out to me was that the highest reading I saw from the MAF was 140 gm/s under full load WOT at 4100 RPM. I saw someone else's post saying it ranged up to 180 gm/s.

I ordered another (used) through Ebay and will have it in a couple days.

I will post the results after I get it installed.

Bill

 

Bill, Sounds to me like you have a misfiring coil (that becomes present upon full warm up) - just went through this in my 00 S Type - same deal - no misfires until after full warm up and no CEL.

If your replacement MAF doesn't fix this issue - dig further on the coils.

If you have a COP tester - test each coil at idle at full warm up - this may assist you to isolate the offending coil.

 

Thanks Abonano

I don't have a COP tester, but could move coils around. For now, I ruled out the coils because it shows no codes and when it starts missing, the power drops off dramatically. To the point that it's better to ease off the throttle to get more power. Also, I would expect the ST and LT trims to go negative with a bad coil due to excess fuel. Mine are almost always positive.

I have a 2001 Corolla that showed the same symptoms when it's MAFS failed. Replaced twice now. Once at about 100k and again at 160k. (I keep my cars a long time...).

I'll post the (hopefully good) results when I get the MAFS changed.

Bill

 

Sounds like a plan Bill - hopefully you will report back with good news...

 

Running cold all the time would have kept the engine in open-loop. Now that it is able to go closed loop it may be using old adaptations. Perhaps a reset is in order.

 

Color me ashamed. I thought I had diagnosed the problem as the MAF sensor.

After installing the replacement (used) MAF sensor, I took a long drive. The misfire returned, and finally went into restricted performance.

When I returned home and put the OBD reader on it, it showed cyl 8 misfire. That proved be be oil in the spark plug well. Argh!!! I had checked several other plugs and not found any oil. The moral of this story is always check EVERY plug for oil.

Since I would have the cam covers off, I went ahead and replaced the upper tensioners using the zip-tie method. Very easy job. I had to wait almost 2 weeks for the gaskets! Parts4Export on fleabay is now on my Naughty list.

Since the gasket replacement there are no more codes and no misfires.

NOTE to all others, always check ALL plugs for oil.

Bill

 

Good job Bill, in fact, very good troubleshooting all around.