Vacuum Line To Ecu

(Greg) #1

So I am almost done replacing all my 30 year old hardened rubber vacuum lines with new silicone vacuum lines.

Final two- the one that goes down to the trans and the one that goes from engine air pipe back to the ECU line.

I’ve been reading that the ECU line connection can only be replaced with the engine out or moved?!? Any tricks to get to this with the engine/trans fully in? Long needle nose pliers I hope? (And due to the heat around that area, I am going to use fuel line as the vacuum hose.)

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #2

I recall that connection as being a PITA, but perhaps no worse than several others on this car.

Why fuel line? Isn’t silicone more heat resistant?

You might note that some have had problems with poor throttle response that turned out to be liquid in that vacuum line. Nobody even knew what liquid it was; could be fuel pooling in there, could be condensation, could be heavier components of gasoline collecting. Whatever, they’d disconnect the line at the ECU and blow through it with compressed air, pushing whatever’s in the line back up into the intake manifold, and the car would run better.

To reduce the tendency for such liquids to get in there, I have proposed removing the balance pipe and flipping it over. This would put that vacuum tap pointing up rather than down. Then the vacuum line would have to make a loop to go back down. I dunno if anyone has actually tried it and I don’t know if it would really prevent the problem, but obviously one would need to ensure that the hose doesn’t kink.

(Greg) #3

Well, a PITA is better than impossible. I’ll give it a try.

I thought fuel line was more heat resistant. I’ll go with silicone if that’s the case.

I haven’t checked that vacuum line for liquid, but I DID note that the two 45 degree 1" hoses on each end of the air pipe had a tiny bit of oil in them. I assume blow by is pushing oil vapor into this pipe at full throttle (when there’s no vacuum), or the PCV valve is clogged. I like you’re idea, but if it’s oil blow by, it will still find a way upwards. And if oil vapor is getting into that line, then I’ll def use fuel line, as oily silicone can collapse.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #4

If you absolutely can’t get that vacuum line connected, you could always go under the car, find a spot you can get to, cut the metal line there, and feed a long piece of hose through and connect it up there.

(Paul M. Novak) #5

I removed and replaced the vacuum hose leading from the balance pipe to the EFI ECU a few years ago on my wife’s 1990 XJ-S convertible (5.3 V12 w/ Marelli ignition). I had to remove the left side intermediate exhaust pipe to access it easily. In my case that was OK because I also had some exhaust system issues to fix so it was no real extra work. I had the car up on one of my four post lifts so access was pretty easy. I did this when I removed and replaced all the coolant hoses, including the heater valve ones, so I had that area forward of the firewall wide open so the job was not especially difficult because every thing was easy to access at both ends of that hose.


(equiprx) #6

When my 94 coupe needed replacement hoses, I am fairly certain that the hose going from the intake manifold and at the ECU are easily accessible both at the ECU and at the firewall.
The one in the engine compartment connects to a steel line near the upper firewall.
It stays steel till it enters the boot under the battery tray.
Other years may have a different setup but I’m sure about 94s.

(Greg) #7

sounds like by 94, they smartly made your connection at the firewall. Our pre-facelifts are around the trans.

How hard is it to remove the intermediate exhaust pipe? That doesn’t have to be removed from the exhaust manifold, does it?

Kirby’s idea is a good backup plan if my trusty mirror and super long needle nose pliers don’t work.

If I can get my hand in there, I can do it. Have done lots of work by touch.


There’s a visual path on the 86 coupe. You can just see the clamp. Shorty screwdriver thin arms both handy, maybe some butter.