As the title says, my '86 XJ6 will start, but it won't rev past 700rpm. To get it to do that I have to push the throttle in. If I let off the throttle, it will sputter and then die. Pushing more or less on the throttle doesn't change the engine RPM.

Here's a somewhat brief history:
Initially I parked the car ~3 months ago to drop the rear subframe out due to a leaking differential. Life and kids intervened and instead of a couple of weeks to replace, it took ~3 months. I did finally get her cleaned up and back down on all 4 tires. No problems with that job.

However, when I went to start her back up (didn't try starting during the 3 months since I had half the exhaust off the car) nothing. I did some testing and found no spark. Further testing pointed to the amplifier or something in the distributor. I had been looking at the Petronix setup from before so decided now was a good time to switch. Got my parts in and just installed them. Car fired right up. It wouldn't rev past ~700, but it did start. The first time the engine did idle long enough to allow me to rotate the distributor both ways and come back to a happy sounding medium. I realize this isn't a scientific timing method, but given the lack of revs I figure that got me close. Then the engine stalled. I got back in, started it up and she fired up, but still wouldn't rev over 700 no mater my throttle position. If I let off on the throttle, she'd stall. I did this 5-6 times. If I hold the throttle she'd keep running, just not revving.

I did put the amplifier back on the end of the intake to make sure there where no vacuum leaks there. I double checked my hoses. Reinstalled the air cleaner and believe everything is hooked up as it should be during normal operation.

Before parking the car, she ran great. Again, the only reason I stopped driving was to fix the leaking diff.

Any ideas? What causes a running engine to not rev past ~700rpm?

Any help is appreciated.

Take care,

Eric

 

My initial gut reaction is "fuel starvation".

But, first.....

Are you absolutely sure the ducting between the air flow meter and the throttle body is intact and properly fitted?

Cheers
DD

 

I'm not absolutely sure of anything at this point so I went out and double checked. Best I can tell the ducting between those two looks good.

I was also leaning the same direction as you Doug. I just removed the hose going into the fuel rail and didn't see as much pressure as I was expecting. Not sure that means anything. At that time the car had been sitting for an hour or so.

I clamped the hoses on either side of the fuel filter and removed it. The fuel coming out was clean and seemed to flow ok. I blew on the end and with about the same amount of air pressure as you'd use when a doctor tells you to breath in an out, I could blow through the filter. Again, I don't have a frame of reference to know if this is ok or not.

I then took the hoses off the fuel pump starting with the outlet side and then the inlet side. Fuel was coming out of the inlet hose very briskly just from the force of gravity. When I hooked the inlet hose back up to the fuel pump no fuel came out the other end of the pump. Does this sound correct? Would no fuel come out the other side unless the pump is working?

That's where I am right now...

Eric

 

Another question, I see an air bleed valve in the overall fuel schematic. Is this something I need to manually operate to bleed the air from the fuel circuit or is this taken care of automatically?

Thanks,

Eric

 

I checked the voltage going into and out of the fuel pump and there was no drop so maybe the pump is good. Still not certain though. Another possibility could be the fuel pressure regulator is not maintaining enough pressure on the system to force enough fuel into the cylinders.

Does someone know what pressure the regulator is supposed to operate at? I will get a fuel injection pressure tester and start testing the system.

Thanks,

Eric

I just did a search on the web and found these 2 links, one of which states the fuel pressure should be 32psi.

http://www.captainjaguarscathouse.co...p_4.2_F.I..htm
http://www.jag-lovers.org/newbies/se...s_iii_xj6.html

 

Well, I got a fuel pressure test kit and installed the pressure gauge between the flex fuel line and the end of the fuel rail closest to fire wall. I ran the pump without starting the (had the shifter in R) and got 36psi. I then tried firing the car up and she started right up, maintained 36psi and would not rev past 700rpm again. I had to keep my foot about halfway into the throttle to keep her running, but modulating the throttle pedal didn't seem to affect the engine. If I let off the throttle she would stop running.

I'm at a loss for what to do next. Any ideas?

Eric

 

After stepping back and trying to collect my thoughts I decided I should have another, closer look at the boot between the AFM and elbow so I went ahead and pulled it all the way off and inspected it. It looked ok and the rubber was still pliable so I reinstalled, making sure I got it on properly on both ends. I then fired her up with the same results.

I then grabbed my wife and asked her to start it so that I could look at what the AFM was doing and hopefully test some of the connectors. Well she could only keep it running long enough for me to see the barn door opening about halfway and holding fairly steady so I at least know it is doing something, but she couldn't keep the car running long enough to be able to test anything out.

Help?

 

No, no action needed on your part


Cheers
DD

 

36 psi regulated f/p is good

Cheers
DD

 

It's beginning to sound like a faulty AFM or ECU. The system apparently isn't increasing injector pulse width to coincide with increased throttle opening. 'Tis the AFM that commands the ECU to do this.

The top of the AFM can be carefully pried off/cuff off so you can inspect the innards for an obvious fault.....like a broken resistor track. Usually, though, substituon of the AFM (and ECU as well, for that matter) is the diagnostic method used by most DIYers.

Can't hurt to make some wiring checks at the ECU. Any corrosion at the big connector? AFM wiring and connector in good shape? The F.I. grounds all come together at the rear of the water rail. You'll see the bundle of black wires under a bolt head. Check for tightness and cleanliness.

Pull the connector off the coolant temp sensor and inspect. If OK, jump the terminals inside the connector with a paper clip. Any change in how the engine behaves?

Cheers
DD

 

All the injectors with L-Jetronic fire together, twice per engine revolution, and the car can run with only one or two injectors firing but obviously wont give any serious power. Can you hear them all clicking OK ? To start the car, there is another injector for cold starts that absolutely floods the inlet manifold with fuel, so it could be running on this for a time, but will eventually stop if the main injectors are not working.

The cold start injector is switched off by the Thermotime switch screwed into the water rail. Can you get it running long enough for the engine to warm up so the cold start injector is switched off ? You could of course, just disconnect the cold start injector after engine start, and see what happens. In order for the ECU to fire the injectors, it needs a pulse from the ignition system, and this is taken off the coil.