Car still idles rough/runs rich after 2 months work - I'm at a wall

(Carl Hutchins, Jr. ) #204

Gee, Greg, try the easies;

  1. Remove the air cleaner and see if the rich issue dissipates.

2,Invest for another nice tool A fuel pressure guage. One intended for house water pressure will do. from the local hardware. Plus some fittings to adapt to the fuel rail…

See olden J-l liter Alex Carnara’s posts.

Extra comment:
A. Adding RPM’s will mask a :miss".
B. cylinders that do not fire at low throttle may fire at more throttle,. A compression issue.

Carl ,

(WayneC) #205

check out this eBay listing, they say no charge if your unit tests as OK:

(Greg) #206

I did test with air filters removed, no diff. They are new too.

The car doesn’t just run rich, it runs extremely rich, so much that I can’t let it idle more than 10 minutes or my eyes start hurting.

I need to replace fuel fouled plugs soon, so will do a second compression check then. Remember, in Oct they were 180-195 cold. Car has 48k miles.
I had a fuel gauge, but it broke. I borrowed one last week, fuel pressure was 36 before start and 33 at idle with 10 inHg.

Thanks for the ecu repair link, I’ll take a look!

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #207

There have been a couple of discussions on here about people repairing their own ECU’s by simply replacing all the electrolytic capacitors with new. Apparently those things don’t age well.

(Steve) #208

IIRC, there are many outfits throughout the US that claim to be able to work with the Lucas ECU for our V12 Jaguars, yet, I do not recall anyone on this list ever (successfully) using them. The AJ16 engined cars are probably the exception here.

The ebay listing has over 580 stars from the last 6 months, none from a Jaguar…

(Paul Wigton) #209

They don’t: they dry out. I got reasonably good at doing so, on old tube radios and Round tube TVs.

(Jimandhelen) #210

The capacitors in the Ecu are tantalum, supposed to be far superior to aluminium caps.



(Greg) #211

Just making sure - I removed a/c, air pump et al, and cruise control a while back.
Having these not giving feedback to the ECU wouldn’t cause any problems?
Besides a bad ECU, why would the injectors not adjust their pulse width correctly? Can a bad resistor pack do this? Can a bad ground to the injectors?

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #212

Don’t forget you have a reported 10ins vacuum.
Even if the ECU, all sensors, all injectors and anything else you can think of is 100% perfect you are going to have much wider injector pulse.
However, we are not sure if the 10ins vacuum is a reason or a result. I would opt for result but case not proven.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #213

The photo of the ECU in this thread shows two orange tantalum caps.
Plus a number of film type caps.
Going back 30 or 40 years those tantalums were not so reliable, they could just fail for no reason even if voltage rating well above working voltage. Reliability has improved since. Modern aluminium electrolytic capacitors are quite reliable too.
Yet again, ECU failures are relatively rare.

(Greg) #214

So even though I just got my injectors cleaned and tested, I decided to measure their resistance. I tried a few of them, and they are all coming back around 2.8 ohms, is that good?

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #215

Injectors are low ohms, can vary a bit from type to type.
2.8ohms is fine.

There is nothing wrong with your injectors.

(Greg) #216

The book said an improperly grounded resistor pack could cause voltage spikes back at the ECU.

Correction: The book said an improperly grounded injector harness could cause voltage spikes back at the ECU.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #217

That has me puzzled. Captain Cathouse ECU circuits shows the resistor pack with 8 wires, non of them ground. It has 4 wires orange plus stripe that are direct to ECU, and they switch to ground in the ECU to give full voltage across injectors. This provides high initial current to snap the injectors open. There are 4 wires pink plus stripe that the ECU switches to ground thru the 4 resistors in the pack.
The ECU switches off the orange wires, switches on the pink wires for holding current.

The pre HE had a so called Power Amplifier to drive injectors, separate from the ECU. That had a ground connection. If it had transistors switching to ground it would need a ground connection.
It is 30 years or more since my 1979 coupe had the OEM ECU so I don’t recall all the detail.

(Philip Lochner) #218

AC and cruise control have NO direct effect on the ECU and having removed these, have no influence on your symptoms.

Injectors have no intelligence of their own. The ECU controls them and there are only 6 inputs to the ECU that determine what happens to the injectors:

  1. By far the most important is the vacuum signal from the centre of the cross-pipe that goes to the ECU;
  2. The remaining 5 inputs are: Coolant Temp sensor, Intake Air temp sensor, full load enrichment switch, battery voltage and the throttle position sensor (mounted below the throttle pedestal). TPS only comes into play when the throttle is actually moved.

Think of the injectors like a relay. When you switch the ignition on, the injectors will have 12V on one of the two pins - like a relay. When the ECU wants them to squirt fuel, the ECU will ground the other pin - like a relay. So, the injectors have no ground of their own. Its the ECU ground that comes into play here, but a bad ECU ground won’t make the engine run rich, it will run lean - or not at all.

There is one major difference between OEM JAG V12 injectors and a relay (from an electronic perspective): As you have measured your injectors, they have a resistance of only 2.8Ohm. These are called “LOW IMPEDANCE injectors”. A relay’s resistance is MUCH more an can remain active indefinitely. But a 2.8Ohm injector switched to directly to ground (via the ECU) is at risk of being fried. THIS IS WHY there is a resistor pack. A low impedance injector switched to ground can end up drawing I (Amps) = V (Volt) / R (Ohm), I = 14 / 2.8 = 5A per injector. If we now calculate the Watts for this injector we get W (Watts) = I x V = 5 x 2.8 = 70 Watts. Think of how hot your 55W headlight lamps get to put that 70W into perspective…

So what happens now is that the ECU has TWO ways of switching an injector to ground:

  1. one route is directly to ground;
  2. the other route is also to ground BUT, via a resistor (inside the resistor pack). The resistor reduces the amount of current flowing (thus preventing it from over heating) but its enough to keep the injector open.

When the ECU wants an injector to open, it will switch on BOTH routes simultaneously but after about 1mS (0.001 S), it switches the route directly to ground off, thus leaving only the route via the resistor pack.

The route directly to ground is to open the injector as quickly as possible and the route via the resistor pack is simply to keep it open.

On the jag the injectors are grouped in sets of 3, so 3 injectors are supported by 1 resistor.

So to answer your questions:
"why would the injectors not adjust their pulse width correctly? "
They only do what the ECU tells them to do based on those input parameters.

"can a bad resistor pack do this? "
No, a bad resistor pack can only reduce or break the current to a group(s) of 3 injectors causing them to run lean or not at all.

“Can a bad ground to the injectors?”
A bad ground can affect the injectors INDIRECTLY through one of the 6 input parameters or directly by not allowing the intended current to flow through the injectors via the ECU to the ECU ground.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #219

Yeah, I wondered about that, too. I don’t recall anything grounded in that resistor pack. IIRC, there was a discussion about a ground that was near the resistor pack, but I dunno how any ground there could damage the ECU.

(David Jauch) #220

In any case, we can assume that the injectors work exactly as they should. They are tested and cleaned. The car runs, albeit rich, so the injectors open and close just as they should.
Just as Richard said the vacuum is too low. That in general would indicate a leak and with higher manifold pressure (due to the leak) more fuel would be injected or it would default to something. (With most cars obviously a vacuum leak leads to lean running as soon as the leak is behind the throttle. With a pressure sensing system instead of a mass or volume metering unit ahead of the throttle the ECU should assume more air, so more fuel is needed). I would default to seeing an assumed vacuum leak as a reason for lack of vacuum. Is it possible to isolate the two banks and compare the difference in pressure if there is any?
If the pressure was identical I would then agree with Richard and try to find the reason for low vacuum without a leak - timing? Advance?

Injectors and grounds are important, but I think they distract from the issue.

(Greg) #221

thanks for all the info about injectors and resistor pack, I really appreciate that. I’m getting a crash course on the V12, and I need it. I mean the ground on the radiator frame next to the resistor pack. I thought it was for it, i think it’s for the injector harness??

Regarding vacuum, that’s where I was a week ago, looking for vacuum leaks and verifying timing. No leaks, and timing was good. I then verified that the engine vacuum was properly reaching the ECU. At 10 inHg, I then disconnected the vacuum line to ECU, no change at all. I then supplied a fake 20 inHg to the ECU with a vacuum pump, no change at all.

So I am not sure the injectors are opening and closing as they should, the ECU could be telling them to run as if there is 0 inHg. Car runs painfully rich - fouls plugs almost right away, oil turns black within a few days, tons of soot coming out of exhaust pipes, and my clothes and eyes are paying for it. I also am going through gas at an alarming rate, just idling.

I am starting to lean towards bad ECU too.

(Greg) #222

Why I am looking at grounds, From The Book:

EFI GROUNDING PROBLEMS: Michael Neal sends this experience: “…the radiator had been replaced and a ground for the fuel injection harness at the right front of the engine compartment had not been re-secured. It had blown two ECUs before I found this. I don’t know why but it took the ECU a week or two to blow. After I replaced the ground and ECU there were no further problems.”

Reportedly, if the engine clearly is running very rich or possibly won’t even start because it’s too rich, it’s time to start looking for grounding problems in the EFI wiring. John Napoli relates one experience: “Although ground to the ECU was one of the many things we had checked early on, the owner later went further and checked the ECU connector. He found an intermittent ground on pin 1 at the connector. This pin, according to our schematics, should ground to the frame of the car at the ground points near the battery. This wire was identified in the loom and a splicer to ground added. The car started fine and at this point is OK.”

If you’re still reading, I took a look at both my grounds on left and right side of radiator. Both bolts had no continuity!? They weren’t rusted, but touching head to thread with ohmeter, nothing. The washer touching the radiator frame wasn’t grounding due to corrosion. So I have since replaced the bolts with conductive bolts, so the threads supply the ground through the bolts. Now they have perfect ground.

(Dzia) #223

This is what I use to ensure good grounds. They are called bonding brushes. Chuck it in your drill and spin until a nice bare metal spot appears.