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Slight miss at idle- barely passing emissions

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@dam

NAXJA Forum User
Vehicle: 1995 XJ Sport, 4.0, 5-spd, 230k miles
Gets low 20's on the highway and runs smooth
At idle, there's a slight miss and you can smell it's running rich.

Anyway, the miss has been there for ages, and emissions get a little worse every year. We got it to squeak by by revving it up to 1100 rpm for the idle portion.

I replaced the cap, rotor, wires and plugs (Champion Truck). I also replaced the idle air control motor. None of that made a difference. The tech said the timing must be off, but I believe that's not adjustable on these, correct?

Any other ideas? The coil is the only ignition part left to replace. Not sure how to adjust timing. Perhaps it's a fuel injector whose spray is uneven at low power settings. Any suggestions?
 
Have you done a compression and leak down test?

Hit it with a timing light?


I was having a similar issue (Not saying this is your problem) and it turned out to be a gummy/burnt exhaust valve on cylinder 3. It's always best to do all the tests you can before randomly throwing money at it.
 
Without the actual results from the test, it's anybody's guess.
 
Hmmmm...I've not done a compression test, but mpg is still good as new and emissions clean up at even slightly higher rpm. There's a hill near me where it basically maintains speed at 60 mph in top gear, and it's been about the same there 10 years later. That said, perhaps you're right. If that's the case though I'll probably turn this car this year rather than sink a bunch of money into it. It's a shame; good car other than marginal emissions.
 
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HC was at 155 ppm at 1100 rpm, but only because we revved it up first. Turn it on and let it idle and it was like 500 ppm at 700 rpm, which is normal idle speed. At 2500 rpm it was 70 ppm- high, but well within acceptable passing limits of 180 ppm. CO and CO2 were similar at all speeds.
 
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Tested and replaced the Injectors?
 
do you have timing chain rattle from up front ?
or has the timing chain been replaced ?
230k your timing chain might be stretched. causing all these things.
 
I've not tested the injectors.

How do I check the timing chain? I don't hear a rattle. Sounds like quite a job do replace that. Power and mpg are still good.
 
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Timing is not adjustable, correct.

I would get a meter out and test the O2 sensor wires, O2 sensor and if the 95 had one, the heater power relay for the electric heater element in the O2 sensor. O2 sensors can get funky at idle and make the ECU switch to rich open loop mode at idle if the electric heater in the O2 sensor is not getting power and working at idle.

If the 95 still had the old Renix vacuum controlled fuel pressure regulator FPR, on the fuel rail and not in the gas tank, it can go bad and dump fuel into the vacuum line making it run rich at idle.

THose are the first two common issues I would check for. You may have a poor sealing injector (dirty or worn seal) at idle.

Do this test, disconnect either the secondary spark plug wire, or the fuel injector power connector one cyl at a time, and when you do the idle should get worse. If one of the cyls does not make the idle worse when you do this, then you know which cyl has a problem. That eliminates or isolates the problem to one cyl. or things like the O2 sensor or the FPR.

If it is one cyl, then do a compression test on it. If compression is OK, then pull the fuel rail and test the injector, more on that later if you get that far.

Lastly, one cyl that does not make as much difference (does not make it run as rough as the other 5) when you cut its fuel or spark, could be a vacuum leak on that one cyl at the injector o'ring seal in the intake or a loose manifold bolt on the side...!!!!
 
Don't worry about the timing chain.

I have had rigs up to 300,000 miles with the original timing chain with no issues. When they go bad it will be obvious!!!!

Lastly, from the sound of it, it is very minor problem don't give up too soon on it. Injectors are cheap, used, clean and tested balanced sets on ebay are supper cheap. The FPR (if 95 had one on the fuel rail is cheap and an easy swap) O2 sensor are cheap....

Also be sure to cheek that every single vacuum hose is good, new like and has a solid perfect seal. If not replace them. They can cause idle speed hunting, wander and run it rich at idle too. And is the air filter clean?

How old is the Cat Converter???? Miles on it??? It may be slowly going bad.

Lastly, many folks recommend running a bottle of injector cleaner, like gum out or BG-??? in a tank of gas before inspections.

Lastly, I think most states exempt 25 year old rigs from sniffer tests. You only have 3 years to go!!!

@dam said:
I've not tested the injectors.

How do I check the timing chain? I don't hear a rattle. Sounds like quite a job do replace that. Power and mpg are still good.
Click to expand...
 
There may be a problem with the fuel pressure regulator. If it's not working the fuel pressure will be too high causing more fuel to flow during the time the injector is fired. The wrong size injector(s) could do the same thing even if working correctly. The ECU will adjust the injector on time to correct this but only within a programmed range (it has minimum and maximum on time limits for the injectors).
 
Kerry Harrod said:
There may be a problem with the fuel pressure regulator. If it's not working the fuel pressure will be too high causing more fuel to flow during the time the injector is fired. The wrong size injector(s) could do the same thing even if working correctly. The ECU will adjust the injector on time to correct this but only within a programmed range (it has minimum and maximum on time limits for the injectors).
Click to expand...

Sounds like it is an idle miss only problem, that has gotten slowly worse over many years.

Where is the FPR on a 95? Was it still on the fuel rail?
 
If you are getting 20mpg you aren't running too rich. If anything, you may be running too lean.

Get it to the point where it is missing. Pull each sparkplug wire individually and note how many rpms it drops. If one cylinder drops less than the others, that is most likely the culprit cylinder. Do the same with the injectors. If you locate one, swap it to a different cylinder and see if the results follow the injector or stay with the cylinder.

I would rent/borrow a fuel pressure gauge and a vacuum gage and check the fuel pressure and then see if you are running low on vacuum. You can use the carb cleaner trick on all the vacuum lines and around the intake to see if it sucks any in and the rpms change.
 
It does sound like an idle only issue but my thoughts were if fuel pressure is too high it may not be a problem when the engine has a load. A pressure gauge would tell. I'm not sure about 95 but in 96 the regulator had moved to the tank.
 
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Open your hood at night and see if you can see any sparks. It really sounds like a bad set of plug wire's that have been rubbing in the same spot and slowly wearing them out.

Good luck and wires are cheap and easy.
 
Wow- that you guys for all the suggestions. I was out of town all weekend so haven't had time to try them yet, but I'll let you know if I find it.

Regarding vaccum leaks...the rear CCV orifice on the valve cover has a damaged fitting where it connect to the tube. I used RTV and electrical tape to seal it up. Might that be it? Where else should I look for leaks? I gave it a once over and the vac system looks relatively simple. What's the carb cleaner trick?

There is a little device on the front of the fuel rail connected to a vac line. I assume this is the FPR. It has an 8" or so line that goes directly to the intake manifold, so I don't think there's many opportunities for it to leak.

I live at high altitude and do a lot of highway miles. I can get 23+ mpg if I don't go over 60. I think the altitude helps mpg a bit- less drag and the throttle is held open more, which reduces pumping losses. HC emissions were much better, but still a bit high, with the engine at 2400 rpm so I don't think it's running lean.

Catalytic convertor has about 110k on it. The stock one had the core come loose inside and was rattling.

Plug wires are a few years old, but there was no improvement when I swapped them.
 
@dam said:
Wow- that you guys for all the suggestions. I was out of town all weekend so haven't had time to try them yet, but I'll let you know if I find it.

Regarding vaccum leaks...the rear CCV orifice on the valve cover has a damaged fitting where it connect to the tube. I used RTV and electrical tape to seal it up. Might that be it? Where else should I look for leaks? I gave it a once over and the vac system looks relatively simple. What's the carb cleaner trick?

The CCV orific line a controlled leak of the crank case, a vent, to the intake manifold. You should buy a new one and replace it. Crap builds up inside and restricts flow, and any leaks make it worse.

There is a little device on the front of the fuel rail connected to a vac line. I assume this is the FPR. It has an 8" or so line that goes directly to the intake manifold, so I don't think there's many opportunities for it to leak.

YES, that is the FPR, but if it goes bad it leaks gasoline into the vacuum tube-hose and screws with the idle!!! Of it is not regulating pressure at the right pressure, Pull the Vacuum hose off after you turn off the engine and see if there is any odor or Liquid gasoline there at the metal nipple or inside the hose.

I live at high altitude and do a lot of highway miles. I can get 23+ mpg if I don't go over 60. I think the altitude helps mpg a bit- less drag and the throttle is held open more, which reduces pumping losses. HC emissions were much better, but still a bit high, with the engine at 2400 rpm so I don't think it's running lean.

Catalytic convertor has about 110k on it. The stock one had the core come loose inside and was rattling.

Running rich can cause that kind of damage to the Cat.

Plug wires are a few years old, but there was no improvement when I swapped them.
Click to expand...
 
Ecomike said:
YES, that is the FPR, but if it goes bad it leaks gasoline into the vacuum tube-hose and screws with the idle!!! Of it is not regulating pressure at the right pressure, Pull the Vacuum hose off after you turn off the engine and see if there is any odor or Liquid gasoline there at the metal nipple or inside the hose.
Click to expand...

Oh! The engine hasn't run in a few days, and I pulled the FPR vac line and it smells like gas. I ran it for a minute and there was no real change. Guess I need a new FPR and problem solved then, yeah?

This makes a lot of sense. Thanks.
 
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I'm thinking though...upon shut down wouldn't a little fuel leak out of the injectors and fill the intake manifold with enough vapor to produce that smell?

I just let the engine run and unplugged the FPR hose from the intake manifold, and it didn't seem very gassy smelling. It idled a bit faster with the FPR unplugged. Just to see the reaction, I sprayed some throttle body cleaner into the little vac nipple and it almost made it stall.
 
@dam said:
Oh! The engine hasn't run in a few days, and I pulled the FPR vac line and it smells like gas. I ran it for a minute and there was no real change. Guess I need a new FPR and problem solved then, yeah?

This makes a lot of sense. Thanks.
Click to expand...

That sure sounds like a leaking FPR.

@dam said:
I'm thinking though...upon shut down wouldn't a little fuel leak out of the injectors and fill the intake manifold with enough vapor to produce that smell?

I just let the engine run and unplugged the FPR hose from the intake manifold, and it didn't seem very gassy smelling. It idled a bit faster with the FPR unplugged. Just to see the reaction, I sprayed some throttle body cleaner into the little vac nipple and it almost made it stall.
Click to expand...

The increase in idle when you pull the vacuum line is normal. There should be no smell in the vacuum hose when the engine is running as it is under vacuum, but when running you can look at the male nipple on the FPR for signs of liquid gasoline there, where the leak would be.

Cut the engine off, then 5 minutes later pull the vacuum line and look for liquid gas and the smell then. The FPR is still under pressure when the engine is turned it off and the leak would persist with no vacuum to remove it.
 
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