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Engine runs fine at idle but bogs down/fights itself when revving sometimes

XJ6 & XJ12 Series I, II & III 1968-1992

Engine runs fine at idle but bogs down/fights itself when revving sometimes

 
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Old 01-07-2012, 09:48 PM
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Default Engine runs fine at idle but bogs down/fights itself when revving sometimes

I just took possession of a '83 XJ6. There are many problems with it, but one of the problems is that it doesn't drive (duh!).

It seems to be a fuel problem. I need to jumpstart the engine (the battery is dead), but once I do, it idles perfectly fine--alternator seems strong as the headlights look nice and bright once I start it and disconnect the jumper cables). Idle starts off a little high when it's cold, and then settles down nicely to around 1000 rpm.

The first time I tried to drive it, I drove off. It seemed to hesitate a little bit, and then I got up to maybe 10 mph. The engine seemed to start to fight itself (almost as if there was a stuck throttle plate) and eventually the RPMs dropped low and the car just died and went completely silent (since the battery was completely dead).

Do you guys have any idea what this might be? Could it be a clogged fuel injector? Clogged fuel filter? Bad fuel pump? Rats nest in the air intake?

Any way to test any theories?

My theory is that it just needs an Italian tuneup. I tried revving it with varying levels of aggressiveness, and sometimes when I push the throttle really hard, it revs up just fine. Other times, not so much.

It seems to me that parts on these cars are SO CHEAP. I'm used to my '06 Jaguar where any single part would send me into the poor house. With the Series III, it's the number of parts that will need replacing that will send me into the poor house, not the price of any single part!
 
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Old 01-07-2012, 10:41 PM
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I just started another thread stating that my coolant gauge doesn't seem to be reading correctly. Then I started reading other threads with similar issues with fuel injection, and somebody stated that " Coolant temperature transmitter - one of the two big influences on injector pulse width, (the other being mass air flow from the MAF box), the other inputs are just trimmers". If my coolant temperature transmitter/sender is bad, my gauge would read incorrectly (which it is), which would also cause the ECU to mess up the fuel injection, right?
 
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Old 01-08-2012, 12:16 AM
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The coolant temp sensor for the fuel injection is different than the temperature sending unit that operates tha dashboard gauge.

The coolant temp sensor (CTS) controls fueling enrichment while the engine is warming up. It can provide too much or too little fuel.

When operating correctly it is out of the picture at about 180F coolant temp.

You can easily check the CTS with a multimeter. Or for about $20 you can replace it...5 minute job.

Loose/corroded connector at teh CTS can mimick a faulty sensor.

Has this car been in storage for a long time?

Cheers
DD
 
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Old 01-08-2012, 08:29 AM
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Where is this CTS? The car doesn't appear to have been in storage long-- the previous owner averaged about 10k miles a year for the last 10 years.

Thanks so much for your help.
 
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Old 01-08-2012, 08:29 AM
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Also how do I check it with a multimeter?
 
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Old 01-08-2012, 10:57 AM
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Originally Posted by amcdonal86 View Post
Also how do I check it with a multimeter?
Testing the Coolant Temp Sensor

Most accurate:
Pan of water on the stove, thermometer, multimeter leads across the two terminals of the sensor, temp sensing probe of the CTS touching the water.

You're looking not only for correct values but for a smooth change in resistance as the water heats up. No jumps, no dead spots.



Less accurate but easier:
Do the same test with the CTS still installed on the engine. You're looking for the same things it's just that you'll be relying on the dashborad gauge instead of an actual thermometer

Cheers
DD
 
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Old 01-08-2012, 11:45 AM
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Well it appears that the temp gauge does work after all. I guess I'm just not patient enough or underestimated how long it takes for engines to warm up.

I finally got her charged enough that she starts under her own power! I tried to rev her up several times and sometimes she would respond, sometime she would just bog down. But I finally go to go on my first "sustained" drive. I now know how the Wright brothers must've felt when they had their first flight that lasted all of 12 seconds!

Since the engine has a stumbling problem, I had to keep it moving by gentle blipping the throttle repeatedly throughout the whole drive to keep it from stalling.

SO anyhow, I did more tests whilst it was sitting in the carport. I think it's a fuel problem. The engine will only rev up if I give it very little throttle, and very smoothly. If I mash the gas pedal as hard as I can, the engine will stumble and rpms will drop--I think I ever heard it backfire a few times. I'm thinking it's either bad gas, or maybe clogged fuel injectors... because I don't know how long it's been sitting.

There is a horrible suspension noise coming from the front that just sounds awful, but I will have to check that out later.
 
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Old 01-08-2012, 12:39 PM
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Originally Posted by amcdonal86 View Post
There is a horrible suspension noise coming from the front that just sounds awful, but I will have to check that out later.

Alan, maybe not as obvious when your engine won't run properly and you have that to deal with, but the donuts on the shock towers are prone to degradation and the shock rods make such an absolutely horrible noise when they go. Not trying to be obvious-man, just sayin'.
 
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Old 01-08-2012, 01:31 PM
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Originally Posted by amcdonal86 View Post
SO anyhow, I did more tests whilst it was sitting in the carport. I think it's a fuel problem. The engine will only rev up if I give it very little throttle, and very smoothly. If I mash the gas pedal as hard as I can, the engine will stumble and rpms will drop--I think I ever heard it backfire a few times. I'm thinking it's either bad gas, or maybe clogged fuel injectors... because I don't know how long it's been sitting.


Remove the fuel filter and empty the contents into a clean container. and/or cut the fuel filter open. What you see (or don't see) will dictate your next steps.

Report back!

Cheers
DD
 
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Old 01-08-2012, 02:39 PM
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Originally Posted by aholbro1 View Post
Alan, maybe not as obvious when your engine won't run properly and you have that to deal with, but the donuts on the shock towers are prone to degradation and the shock rods make such an absolutely horrible noise when they go. Not trying to be obvious-man, just sayin'.
Thanks. Are you referring to the top hats (upper shock mounts)? I have done this before on another car and it doesn't seem like it's that terrible to do.
 
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Old 01-08-2012, 02:40 PM
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Originally Posted by amcdonal86 View Post
Thanks. Are you referring to the top hats (upper shock mounts)? I have done this before on another car and it doesn't seem like it's that terrible to do.
Is there a procedure for this? I want to avoid having 20 gallons of fuel spilling all over and I've never done any sort of fuel system work before. Also where can I find a good repair manual?
 
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Old 01-08-2012, 02:49 PM
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Also, I am almost 100% sure the gas in the tank is bad, but not sure that that is what's causing the problem. When smelling the gas with the tank lid open, it doesn't smell like gasoline. It isn't sharp like the smell of fresh gas, it smells old and maybe somewhat sweet.
 
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Old 01-08-2012, 04:00 PM
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Drain your tanks using the access via the rear 1/4 panels below the 1/4 bumpers. The access holes are normally blocked by plastic plugs. Once off you will see a large hexagon plug with a much smaller hexagon plug in the centre.

Drain tanks using the small plugs - NO SMOKING !! then take out the large plugs and the gauze suction filters will come out with them. Inspect and clean, then flush tanks through via the filler caps with about a gallon of kerosene or similar. Finally a quick flush with clean petrol (gasoline).

Change the inline fuel filter too. It should be in the engine compartment
 
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Old 01-08-2012, 04:41 PM
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Thanks. Is there a way for me to know how much fuel is in there without my fuel gauge working? I don't think I have a drain pan big enough to hold the amount of fuel that could come out of the big fuel tanks.

Is there possibly a way I could just siphon it out the top? I live in an urban area and I just think the potential for a huge spill is not worth the risk. I realize that there is probably a ton of gunk in the bottom of the gas tank.
 
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Old 01-08-2012, 07:55 PM
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Originally Posted by amcdonal86 View Post
Also, I am almost 100% sure the gas in the tank is bad, but not sure that that is what's causing the problem. When smelling the gas with the tank lid open, it doesn't smell like gasoline. It isn't sharp like the smell of fresh gas, it smells old and maybe somewhat sweet.

The fuel has gone stale. Sometimes it'll smell like turpentine. Also be prepared for rusty gas tanks...common issue. This is where emptying or cutting open the filter comes in.

Cheers
DD
 
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Old 01-08-2012, 08:45 PM
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Originally Posted by amcdonal86 View Post
Thanks. Are you referring to the top hats (upper shock mounts)? I have done this before on another car and it doesn't seem like it's that terrible to do.
Right after I posted I remembered yours is several series preceeding the x300. Sorry...but I guess they probably have a similar arrangment? On the x300, where the front shocks mount, they have a couple of rubberish-donuts on the rod sandwiched between cup-washers above and below the mounting point. When I was shopping, one of the cars I test-drove had some deterioration there and made horrendous noises over the most insignificant bumps.

Just snapped a pic of my X300 and looks like it won't be long until mine's making such noises!
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Old 01-09-2012, 07:55 AM
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Is there possibly a way I could just siphon it out the top?
Yes, of course, if there is a lot of fuel in there it's the best way. I was assuming you hadn't much in. Once it's mostly out, follow previous instructions.
 
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Old 01-09-2012, 09:23 AM
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Default My '82 is has exactly the same symptoms

Originally Posted by Doug View Post
Remove the fuel filter and empty the contents into a clean container. and/or cut the fuel filter open. What you see (or don't see) will dictate your next steps.

Report back!

Cheers
DD

small beads of rusty water came out of my filter when I shook it out
 
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Old 01-19-2012, 02:23 AM
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I have little experience with my 87 XJ6 (just got it for Christmas) but I did have a similar stumbling problem just at idle and the idle quality wasn't the best either.

The problem? Turned out to be several things. First off, the thermostat was stuck open so the engine wasn't getting hot enough. Normally it should be just getting into the green section on the dash gauge and mine wasn't. A new thermostat solved that problem.

Next the CTS was faulty. It would start out OK when cold but during warm it it was sending wild signals to the computer so it too had to be replaced.

Once the thermostat and the temp sensor were fixed the car ran even worse! It seems prior mechanics had played with the MAF meter to compensate for the temp problems and messed it up. I got a second hand meter that was known to work fine and it solved that problem for the most part.

Lastly, the altitude sensor in the trunk of the car was by-passed as it was a known source of issues with idle.

Now it's running fine under all conditions except the idle is still not as smooth as I would like it. New cap, rotor and plugs didn't change the idle much either. I'm going to change the fuel filter next and perhaps I will consider a fuel injector service as the engine has 168K on it and I don't believe the fuel injectors have been serviced in some time.

Two points to pass on that I learned from the mechanic who did most of the work. 1) Many of the MAF meters, even the rebuilt ones don't work entirely correctly. For this reason, you may want to try a few and see which one works best. 2) The ignition modules do fail on these engine and many people replace an internal GM component inside with a new one and reuse the old ignition module. The problem is that this new part may not have correct curve for the 4.2 six and may not perform or smog correctly. If yours has been modified you might want to source another and experiment with it.

Good luck to you. If you end up with a really smooth idle, I would like to hear what you did to get it there.
 
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Old 01-19-2012, 07:25 AM
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Well, the issue right now is that it won't run at all, unfortunately. I'm thinking that since the condition drastically changed after I changed the fuel pump that what's wrong with it now has something to do with fuel pressure--since I didn't mess with ignition or anything like that. Perhaps this is a bad assumption. I really should just investigate with a fuel pressure gauge!
 

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