Hello,

I have an XJ6 S3, 1986 which has recently made it back on to the road after a head gasket replacement.

Engine is running well, but once I am up around 3000 rpm or more, the engine is missing every so often. Quite disconcerting.

Ok, I haven't replaced any plugs yet, and they are probably due for replacement, but I am leaning towards the fuel pump. Why? ... 'cos I can hear a high pitched whine from the boot, at idle, and am wondering if it is struggling to supply fuel as the demand from the engine increases.

Anyway, before I start pulling things out and replacing components that may, or may not, need replacement, I was looking for some thoughts as to what might be some possible causes.

 

Plugs are cetainly a cheap and easy place to start. Give it a whirl!

Excessive fuel pump noise usually suggests a problem and these pump DO eventually wear out. Only a fuel pressure test *while the problem is occuring* will confirm.

Can you describe the msfire a bit more? When it happens does it feel like just one or two cylinders have misfired once or twice....a little "bump bump" feel? If so I'd consider an ignition problem---plugs, plug wires, loose wires at the coil, etc

Lack of fuel pressure, even if only for a couple moments, will generally be more dramatic and of course all cylinders suffer simultaneously. You'll typically hear a backfire thru the intake as the system starves for fuel

Cheers
DD

 

Doesn't sound like a fuel pump issue, which would be more likely to cause an overall lack of power or stall out as it would affect all cylinders. Throw a set of plugs and a cap and rotor at it, also (preferably when a little dark out) spray down the plug wires with soapy water and look for arcing.

 

Thanks Doug and VWtechnician for the prompt responses.

Yes the "missing" is just as you describe .... "....a little "bump bump" feel"

Only happens in the higher rev range, so a plug breaking down? a connection somewhere compromised by vibration? a damaged distributor cap?

OK, so I will start with spark plugs, and check leads, etc.

I will let you know how I go.

( I must admit finding quiet roads to push the car over 3000 rpm has been half the fun! - shhh!)

Rhys

 

Curious..when my engine is running..i hear a hissing type noise coming from my boot....is that normal

 

The fuel pump makes a noise that I'd describe as "zzzssssttt". I guess that could be "hissing".

I think you're hearing a normal sound.

Cheers
DD

 

or your hearing aid needs a new battery :-)) (just kiddin')

 

OK... not the plugs (-$25)

Now for the spark plug leads.... what might be a good way to test these.... replace one with anew one and test.... working may way through all 6 leads?

For the distributor cap, just source/borrow a good replacement and test again?

What other points of failure might there be? I do have a weird little wiring modification that seems really dodgy. I will try and attach a photo.

Rhys

 

Photo attached...

Any suggestions as to why this modification has been made?

 

Plug wires can be checked with an ohm meter, if you have one.

Doubt that you'll find anyone selling a single plug wire these days....you'll probbaly have to buy a new set.

Pull the dist cap and check the contacts for burning, excessive wear, and tiny carbon tracks between the posts. Check the rotor tip for wear/burning.

Hard to describe "excessive" wear here. Send some pics if you can.

Cheers
DD

 

I'm not even sure what I'm looking at :-). It think it might be the fuel switch that cuts off fuel at closed throttle.

Can you tell where the wires go?

Sorry I can't be of more help

Cheers
DD

 

Not sure what it's like in Australia but here in the states plugs, cap, rotor and plug leads are dirt cheap. Like $35-40 for all of it. They're routine maintenance items anyways. Did you do the soapy water test I suggested earlier?

 

Hello again,

I haven't tried the "soapy water" test yet.... sounds like a fun activity for the weekend.

Here are a couple more pictures of the "modification" that has been made.

 

here's some easy and not so easy checks I would do to confirm and eliminate, considering the top of the engine has been disassembled and reassembled:

1) make sure the Air Intake Aluminum Elbow and Air Intake Bellows/Hoses before and after the elbow are well seated and clamps are sealing well. Do not overtighten the clamps or you'll deform the rubber edges of the bellows, just tight enough.

start at the front, by the air filter canister and go back all the way to the intake elbow checking all hoses are tight. While you're at it, remove and inspect the air filter canister and the filter itself. There could be a shop towel in there for all you know.

2) there are 3 Ground wires coming from the main wiring harness connected with ring terminals to the top rear of the cylinder head, passenger side, with ½" bolts, make sure those are tight and the wires are secure in their terminals.

3) there are two Electrical Studs or "Posts" sticking out of the firewall/bulkhead, one on the passenger's side, one on the driver's side, on each side of the engine; a number of thick gage Brown wires are connected to them with ring terminals; The studs have a rubber boot covering the posts/studs;

First disconnect the battery Negative cable; then slide the rubber boot back on each of the posts/studs, then loosen and remove the nut on each stud, and use a brass brush to clean each wire's ring connector, the threaded stud/post, and the nut. Make sure they are shiny brass clean.

4) there is a stainless steel Ground Strap under the passenger side where the engine and transmission meet: Remove the strap at the transmission-to-engine bolt and clean the area with a wire brush, as well as the Ground Strap's contact point, and the bolt, nut, and washer. Shiny clean steel. Reinstall tightly. Do the same on the chassis bolt side of the strap, Shiny clean on both ends and tight and the chassis contact point too. (remove any undercoating or grease at the point where the ground strap touches the chassis).

this exercise will "revive" or "renew" the electricals, make the lights brighter and eliminate any bad grounds that may be causing the problem if indeed they are the cause of the problem; I am not saying this will cure your rpm problem but this is a job that needs to be done every 10 years.

Extreme measures I've done to my '84 XJ:
1) remove the Front Catalytic Converter snd clean the insides, it is probably clogged with exhaust crud by now; Either replace it with a European non-cat exhaust pipe (available in the US), or clean all the stuff inside it, reseal it, pressure test it, paint it, and reinstall it. If you live in a state with yearly Inspections, at least it will look like you have the cat in place. But the car will pass emissions testing without the cats, that's how good the Series 3 4.2 XK engine is.

2) Replace the "Y" exhaust catalytic pipe under the car with a non-cat European "Y" pipe, (available in the US), the second catalytic is probably clogged too. By removing the cats you will experience better engine breathing and recover a few extra horsepower.

3) replace the O2 Sensor if you don't know when it was last changed; 60k miles is about max life for these Bosch sensors, then they degrade the engine's performance.

Good luck.

 

Interesting. Can't make out what the widget is at the end of the wires but it's some sort of modification to the coolant temp sensor.

Hard to say if this was someone's idea of an improvement or simply a way of masking a problem with the sensor...or what.

I'd peel back the tape and undo the wires and see what happens.

Cheers
DD

 

Thanks again, Doug and Jose

... I'll have a go at all of that over the next day or two....

I'll let you know what I find underneath all that tape too!

 

Hello,

Problem solved..... wire from coil was dodgy. Cleaned, tightened and tested.

All good now, thanks everyone for the help, yet again.

 

Why you in the U.S dream about replacing with european parts we consider the opposite.
Anyone know a good source for a new such catalytic Y-pipe ?

They seem extinct, I would like to find one rather than have to weld one up from generic cat parts.
If a supplier can´t ship outside the US I can have it shipped to a Florida freight forwarder.

/gunnar

 

I researching a new exhaust for my Series III - They are generic - but the Y pipes and the Cats are all over E-bay.

 

you could cut it out but i wouldnt that coolant sensor goes to the ecu the previous owner probably put it there to tell the ecu the engine is hot so you
can pull your thermostat and it wont dump fuel