Poor Fuel Economy...Sometimes
#1
09-08-2017, 12:49 PM
Poor Fuel Economy...Sometimes
I have a 96 XJS with the AJ16 engine and I have the worst of all problems, the sometimes problem.
Sometimes I start the car and I drive to work, mostly highways, and I see my normal 20-21mpg on the trip computer. The LTFT confirms that I am being properly fuelled by showing me a low number, like 1.something, or 2.something.
Other times I start the car and I feel more shuddering as I drive, and I see that I am in the 16-17mpg range. My LTFT confirms that with a higher number, climbing past 7+.
How can I even begin to diagnose this one? I seem to get a shudder at 50-60mph and at lower speeds in the city, I lightly set my foot on the gas and the car lurches faster then slower then faster then slower. I've tried to catch some strange readings on my throttle setting, but haven't seen anything strange. I'm now keeping an eye on amount of air being sensed.
What else should I look at?
I've recently replaced spark plugs, refreshed fuel injectors, fuel pressure valve...not sure anything else.
Sometimes I start the car and I drive to work, mostly highways, and I see my normal 20-21mpg on the trip computer. The LTFT confirms that I am being properly fuelled by showing me a low number, like 1.something, or 2.something.
Other times I start the car and I feel more shuddering as I drive, and I see that I am in the 16-17mpg range. My LTFT confirms that with a higher number, climbing past 7+.
How can I even begin to diagnose this one? I seem to get a shudder at 50-60mph and at lower speeds in the city, I lightly set my foot on the gas and the car lurches faster then slower then faster then slower. I've tried to catch some strange readings on my throttle setting, but haven't seen anything strange. I'm now keeping an eye on amount of air being sensed.
What else should I look at?
I've recently replaced spark plugs, refreshed fuel injectors, fuel pressure valve...not sure anything else.
#3
09-08-2017, 05:14 PM
Veteran Member
#4
09-09-2017, 08:29 AM
#5
09-15-2017, 10:24 AM
For the heck of it, I took my (new) spare coil and swapped out Coil #1 and drove to work.
After about 15 miles, when I pulled off the highways, the LTFT on both banks dropped from 10.2 to 3.8.
Right now I'm chalking that up to coincidence. Not gonna touch the coils for now, I'm leaving them as is and will monitor and report back.
After about 15 miles, when I pulled off the highways, the LTFT on both banks dropped from 10.2 to 3.8.
Right now I'm chalking that up to coincidence. Not gonna touch the coils for now, I'm leaving them as is and will monitor and report back.
#6
09-25-2017, 10:41 AM
Problem solved!
I did two things that could have solved my problem:
1. I removed the EGR, cleaned it. It wasn't too dirty, but I brushed off the surface and sprayed the inside with carb cleaner. I also cleaned the holes coming in/out of the EGR cleaner at the engine. The hole going into the intake manifold seemed pretty dirty. Cleaned it up with a wire pipe cleaner from harbor freight, and everything was a-ok. I wish I had seen how bad the blockage was before wire brushing that thing out.
2. I replaced the EGR gasket (it looked pretty mangled) and smeared Hylomar AF on both sides of the new gasket, as well as the face of the EGR and the face of the part the gasket gets put on. Reinstalled and torqued, then loosened and re-torqued down to 19lbs-ft. (Book says 17-20)
All is good. The shudder is gone. My mileage is back where it should be. I will expect to see my LTFT drop back to a consistent value now.
I'll post back if something new pops up.
I did two things that could have solved my problem:
1. I removed the EGR, cleaned it. It wasn't too dirty, but I brushed off the surface and sprayed the inside with carb cleaner. I also cleaned the holes coming in/out of the EGR cleaner at the engine. The hole going into the intake manifold seemed pretty dirty. Cleaned it up with a wire pipe cleaner from harbor freight, and everything was a-ok. I wish I had seen how bad the blockage was before wire brushing that thing out.
2. I replaced the EGR gasket (it looked pretty mangled) and smeared Hylomar AF on both sides of the new gasket, as well as the face of the EGR and the face of the part the gasket gets put on. Reinstalled and torqued, then loosened and re-torqued down to 19lbs-ft. (Book says 17-20)
All is good. The shudder is gone. My mileage is back where it should be. I will expect to see my LTFT drop back to a consistent value now.
I'll post back if something new pops up.
#7
09-25-2017, 12:36 PM
Senior Member
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#8
09-25-2017, 03:36 PM
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RickE (09-26-2017)
#9
09-27-2017, 07:26 AM
Hylomar was a bad idea. It cooked right off the engine to gasket side. Oddly enough, the asked to EGR was still thoroughly adhered together with the Hylomar.
I did exchange my 3 year old EGR that I bought from AdvanceAuto for a new one, under lifetime warranty, no hassle since I was able to pull the receipt off my purchase history on their website!
I also bought a tube of Ultra Copper RTV and smeared it on both sides of the new EGR gasket, tightened the bolts finger tight for an hour, then came back after an hour and torqued it down to 19 lbs-ft and drove home!
Mileage is back up to an average of 24mpg for mostly freeway driving! This issue, I hope, has been resolved permanently.
I did exchange my 3 year old EGR that I bought from AdvanceAuto for a new one, under lifetime warranty, no hassle since I was able to pull the receipt off my purchase history on their website!
I also bought a tube of Ultra Copper RTV and smeared it on both sides of the new EGR gasket, tightened the bolts finger tight for an hour, then came back after an hour and torqued it down to 19 lbs-ft and drove home!
Mileage is back up to an average of 24mpg for mostly freeway driving! This issue, I hope, has been resolved permanently.
#10
10-03-2017, 10:18 AM
#11
08-17-2018, 04:09 PM
Finally got to the bottom of this!
After driving for too long of a time with the LTFT at 10.something, it was the TPS that was the root of the problem!
The car drove fine, it just had really bad fuel mileage. No check engine lights, no smoke, not really any other symptom, other than poor fuel economy.
I replaced the TPS, with a brand new one, a week ago and my LTFT is at 0.0! I had a spare old TPS that I inherited as part of a used bin of XJS parts and it did not help. I will take this TPS apart and see whats doing.
I do want to note that the new OEM TPS no longer carries the Ford branding that the original parts bore. Also they come with silver pozi screws, instead of brass torx screws. The plate that holds them in also is silver in color, not the old brass color. Thought I would mention that. It was purchased directly from JDHT.
After driving for too long of a time with the LTFT at 10.something, it was the TPS that was the root of the problem!
The car drove fine, it just had really bad fuel mileage. No check engine lights, no smoke, not really any other symptom, other than poor fuel economy.
I replaced the TPS, with a brand new one, a week ago and my LTFT is at 0.0! I had a spare old TPS that I inherited as part of a used bin of XJS parts and it did not help. I will take this TPS apart and see whats doing.
I do want to note that the new OEM TPS no longer carries the Ford branding that the original parts bore. Also they come with silver pozi screws, instead of brass torx screws. The plate that holds them in also is silver in color, not the old brass color. Thought I would mention that. It was purchased directly from JDHT.
#12
08-18-2018, 03:32 PM
#13
08-19-2018, 08:03 AM