Thats the thing it idles like a champ. You would thing that all is well. but once your foot hits that pedal.....blah.... . I had an 87 series III.

 

If ya aint gone bald yet, you soon will be.

Are there no codes/messages on the VCM?

I had a cracked wire to the temperature sensor on the 92, couldn't actually see anythng wrong, but it did cause the car to go into limp mode. ie no accelerator pedal.

This was when I first got the car, took me a while to figure out what a Fuel Fail message had to do with a duff water temp sensor connection.

Anyhow have a close look at the wire and connector to the temp sender. For my money it is badly located and exposed to accidental damage.


You are not alone in this, CrossheartXJ6 is also having a similar nightmare over in the XJ6 section.

Stick with it.

 

No codes on the VCM. No limp mode. it starts to fail at about 1500 rpm under load. or at a little more then a quarter pedal. if i go soft on the pedal without a load it will go over 2000rpms. but i don't want to hurt my forward gear... what it does is kinda like a misfire into the intake.. pop! pop!.... i've been tring to figure this out for a month and a half. I work at a dealership as a parts guy and all my techs have said " it might be (insert nissan issue here) but so far no good. this forum is my last hope.

 

So as it stands at the moment, the only part not swapped out or checked is the engine temperature transmitter. You also have black plugs indicating over-fuelling, and this component is one of the two major inputs to the ECU for injector pulse length. As far as I can remember, the thermistor inside the transmitter reduces in resistance as it warms up. If this does not happen, or there is a major resistance somewhere in the circuit, the ECU will overfuel the engine, even on tickover.

When did the engine last run properly ? Was it before you did some item of work in the engine bay ?

 

I bought the car like this. about four months ago (so i was told) the car started running rough. he took it to a hippie chop shop where he was told it was the fuel pump. so they "replaced the pump". they replaced the internal pump with one from some after market shop.. it looked like the walbro pump that come in it anyway even said walbro on the side.in the process they thought it would be a good idea to cut a hole on the bottom of the old pump case so that the aftermarket fuel pump filter could stick out...... i replaced the case and pump. it had a FF39 that was the egr silonoid , fuse 1, fuse 2 code. all that was taken care of. SO i got the car back to where it was before it went into the shop, at least i think. It was owned before him by a guy who only serviced a jaguar of south carolina.

 

I might have missed it in the 3 pages but have you confirmed the timing is properly set yet?

 

Yes sir. i check #1 at TDC to the rotor button. looked good.

 

Ok.............Lets have a recap

It starts and ticks over OK, but when you open the throttle it dies on you. So as it ticks over the valve timing is OK, it starts OK, so the temperature sensor looks to be OK (I think !!). So why does it die ?

In your early post you said: -

What do you mean by "MAF controller" ? The MAF itself ? Because when you open the throttle, the partial vacuum in the inlet manifold is eased off and air starts flowing in quantity. Now if this air flow is not measured correctly, there will be the wrong increase in pulse length at the fuel injectors, so the engine will be trying its best with the fuel supplied, but it's not enough for the amount of air drawn into the cylinders. (or maybe too much)

So it seems to me you may have a MAF problem, or a problem with the wiring link between it and the ECU. However, I would have thought this would have shown up as a fault code because the air flow value would not relate to the throttle position. However, as a 91, it may not have the sophistication we expect nowadays.

I assume other things are OK like fuel pressure at the rail, injectors are clean, and spark is at the right moment

 

yes the MAF sensor itself. i got a "known working unit" from the bone yard. i could pull apart the old one to check to see if the wire is covered in nasty stuff. The plug has the gold plated connecters on it.

 

But yes.. its starts on the frist turn of the key. idles fine. open the throttle and its boggs down and does a weird backfire through the intake. Does not die. stitting at idle the timing is right.

 

Check to make sure that the diaphragm hasn't got a pinhole leak in it (just pull the little vacuum hose off and look for fuel dripping out of it). My VCM was throwing a Code 23 (mixture rich) at me when mine started to go, but who knows? It's a very easy check.

 

tried that. no leak.

 

So i took apert the old MAF sensor. (a Lucas/Hitachi 5AM) it was a lucas remanufactured unit. cleaned it even though it looks clean and adjusted it ALL THE WAY DOWN. i guess i'll install it tomorrow.

 

How's about a pic or two of the innards of your opened MAF, not everyone will have seen the inside of one.

One question, with all that has gone on before, do you let the car come up to temperature, or are you now at the point where you are going mad, and starting the car and imediately giving it gas?

 

We disussed the TPS way back, and you say you have cleaned it as there was a lot of gunk in that area, good.

The backfire under acceleration is usually related to "lean "fuel" condition. Is the TPS set at the correct readings at idle??, I think 0.6v (memory not good), and then with the probes still in place in the same 2 wires make REAL SURE that you get a nice smooth rise in volts as you open the throttle all the way to wide open. This is done with ignition ON, and engine OFF. If the readings rise and fall as you open the throttle, this is giving the fuel map inside the ECU odd signals, and will give a lean fuel condition.

Our 3.6 XJ40 had similar issues, and the fuel pump relay was causing some of it known early car concern, BUT the TPS repalcement fixed it 100%.

 

Memory just kicked in again, bugga.

Seperate the plug that the TPS wires go to, the large one just under the intake elbow, it is known to get oil residue from the intake elbow coating it, and water ingress as well, and the terminals get a high resistance, thus giving odd readings to the ECU again.

Again, our 3.6 had "green" growth in there, from washing the engine bay I suspect.

 

The fuel pump relay? i guess i could swap a relay into it to see if it helps....And i'll check the plug and the tps and go from there.

 

I already put it all back together. but i'd be more then happy to pull the old one apart and take some photos. I think i have a 3AM that i can rip apart as well...

 

a very little dirt in the connecter, dry and no green growth.

 

ok... so i while i was messing with car, i was lisening to the exhaust and while its sitting there idling it putts a little. Granted i tried to take the TPS off the car from underneth and it move so it is idling very high now ( i'll fix that) but i just thought i'd let you know.