Help with a few Vacuum Mysteries

I am working on getting my '89 XJS convertible put back together, and I am realizing I did a terrible job of tracking the vacuum connections. I have been trying to use the diagrams under the hood, but there are a few things I can’t seem to figure out. I apologize if these are dumb questions or answered elsewhere, I have tried to find answers in the forum, but to no avail. Here are the questions:

  1. On the back of the right intake, there are four vacuum ports. The diagram notes one is for the ECU, but when running some air through the line from the trunk, its clear the line is attached someplace else as the air runs through the vacuum system. I also saw on some forum posts that this connects to the crossover manifold pipe, not at the right side intake. Should I just plug up this port? Is there a second vac line to the ECU?
  2. Does anyone know what the vacuum port on the front of the left intake manifold should connect to? See pic:
  3. There is a line coming off the canister on the left side that has a T and two short hoses. I can’t find where this one should connect either. Could be related to question 2, but i am then not sure where the second connection might go. Pic:

Thanks for any help here, I really appreciate it!

  1. There’s supposed to be a vacuum line to the Marelli ECU. I dunno if this is where it connects.

  2. RH FPR? I can’t tell from the photo if one of the other lines connects to the RH FPR.

  3. These might be involved in the fuel tank vent recall. There were two hoses connected to the LH throttle body, I dunno if this is one of them.

Welcome to the list. I can help with some of this as I did a major engine bay restoration of my wife’s Left Hand Drive 1990 XJ-S convertible (5.3L V12 w/ Marelli ignition) a few years ago and removed and replaced all engine bay belts and hoses.
I think that 1989 was the transition year from the Lucas to Marelli ignition system. You didn’t mention it but my reply assumes that you have the Marelli ignition system. Attached is a picture of the vacuum lines coming off of the rear of the right intake manifold on our car (which has Anti-Lock Brake System (ABS)).

The small brown vacuum line goes to the Marelli Ignition ECU located under an access panel in the forward outboard corner of the right footwell in our Left Hand Drive car. You will have to remove the carpet, padding, and a couple of bolts to access the Marelli Ignition ECU. The Lucas EFI ECU is a different component completely and is located in a void over the right rear wheel. It also gets a vacuum signal as you mentioned.
The vacuum hose routing in our 1990 is very complicated but it is detailed on the two Vacuum Routing Diagrams on the underside of the hood. Attached are pictures of those diagrams, EBC2681 and EBC2682. All of the info you need is on those two decals, but I will admit that it isn’t easy to decipher. I remember drawing it all out on a big sheet of paper to keep track of what went where.

The nipple on the front of the left intake manifold is part of the emissions system which has many components as detailed on the decals and in the XJ-S Repair Operations Manual and Parts Catalogue.


Thanks Paul! I found the brown tube, I was looking for a black one, so completely overlooked it! I think I figured out the split hose went on the intake nipple, and the other end connected to the temp sensor below it. Everything now seems to be connected, and she is purring nicely, if not a bit loud since this weekends job is getting the rest of the exhaust installed! Thanks again!!