Interesting '93 Jaguar XJ40 throttle issue

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Mar 2, 2004
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Kentucky
I have a buddy that owns a 1993 Jaguar XJ-40 (4.0L I6) that I commute to work with a couple days a week. He drove today and it was an interesting ride. A couple miles from my house, he notices the engine is throttling by itself, keeping a steady rpm and 50mph speed without any throttle input. We have a light highway drive ahead of us for the next 10 miles or so, so he proceeds with caution. Caution soon turns to worry because the car requires riding the brakes to maintain speed down a hill, and the first stoplight we come to required an excessive amount of brake pedal effort. Shift into neutral coming to the stop, and engine races to 4500 rpm. We pull into the next gas station, shut it off, pop the hood and examine things.

Pretty simple idle control system-- single electric idle air control valve that is controlled by the ECU using various inputs. Work the throttle by hand and pedal, smooth as butter. Start it back up, it purrs like a kitten at normal idle, work the throttle thoroughly, check for obvious signs of vacuum leaks. Saw absolutely nothing that would explain throttle sticking / high idle RPM and it's working great at this point. So we proceed on and have no further trouble on the way to work.

On the way home, the throttle issue returned, but was intermittent. After a period of completely letting off the throttle, it would return to normal, but as soon as you were on the throttle again, it was back. We tried unplugging the IAC valve, no change. He nursed it straight to his house. Turning into his driveway it was doing it (had to hold the brakes hard), so first thing I thought was pop the hood right away and let me see throttle position. Problem stopped before or as we got the hood open.

Long story short, we were both befuddled and ran it around the block a couple times, hoping we could pop the hood and take a look while the problem was occurring. The final time we attempted it, as soon as he popped the hood (with me on the side of the car ready to take a look), it went straight back to normal idle. Luckily he made the comment that it stopped as soon as he pulled the hood release, or I would have never put two and two together.

I studied the underside of the hood closely (it's one of those clamshell style hoods) and noticed the noise/heat insulating blanket thing was draped down (they all do it after enough years), but Jaguar affixed a large 10"x8" or so metal plate to that insulating blanket. The metal plate identifies the engine, warnings to not stick your hands in the fan blades, A/C charge, the sort of stuff that most manufacturers put on a sticker. That metal plate was causing the insulation to droop. When I looked at the metal plate closely and the amount of droop, it was clear that it would be resting on the throttle lever/pulley which sits right on top of the throttle body. Where the throttle cable connects to it, it leaves a metal stub at the top, which rubbed on that metal plate long enough that it eventually bore a hole through it. That little metal stub would get stuck in that hole in the plate, holding the throttle open. Just pure coincidence and good luck that we found it.

I'm relieved I made it home safe, and my buddy is relieved he has a very easy fix!
 
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We installed a DIY cruise control kit in an 84 vanagon which we bought from western auto a year or so after owning that van. i kept that cruise control and installed it in 2 other cars along the way. The vac diaphragm pulled a cable which pulled a chain. That chain caused issues in the van. It was there to allow slack in the line if you throttled to pass; as the cruise would sense the overspending and provide additional slack to the already slacked line. That chain would droop, bounce around, and stick to something. You’d take your foot off the gas after passing but the throttle would be hung in “passing“ position. We figured out how to regulate speed with the ignition key until figuring it out. Took us a while…. but subsequent re-installs of that cruise control kept that in mind.
 

92saturnsl2

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Joined
Mar 2, 2004
Messages
3,504
Location
Kentucky
We installed a DIY cruise control kit in an 84 vanagon which we bought from western auto a year or so after owning that van. i kept that cruise control and installed it in 2 other cars along the way. The vac diaphragm pulled a cable which pulled a chain. That chain caused issues in the van. It was there to allow slack in the line if you throttled to pass; as the cruise would sense the overspending and provide additional slack to the already slacked line. That chain would droop, bounce around, and stick to something. You’d take your foot off the gas after passing but the throttle would be hung in “passing“ position. We figured out how to regulate speed with the ignition key until figuring it out. Took us a while…. but subsequent re-installs of that cruise control kept that in mind.
Sometimes the hardest problems to solve end up being the simplest things.

I actually suspected the cruise control right from the start-- a vacuum diaphragm operates the throttle cable, which is controlled by an electric solenoid. It would be an unlikely cause, but in this case nearly all the ECU / engine electronics are made by Lucas-- that's reason enough to be suspicious. I pulled and plugged the vacuum line going to the diaphragm as one of the first troubleshooting steps.
 
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