Vacuum Leak at FI Intake a somewhat surprising location!

Dec 2023  R Kwas  [Comments added]

Vac Leak at Manifold, Occurrence 1
Vac Leak at Manifold ,Occurrence 2
Occurrence 3


Reference Information  
    Manifold Plug Removal
    Luigi Galvani


I have recently become aware of a new and unusual location of a galvanic issue.  It has occurred on two different D-Jet equipped Volvo 1800Es, and since it is not a "one-time occurrence", I thought I would make owners aware of it by documenting it is a vacuum leak, and not in the "usual places" (like tubing, gaskets, or components)!  Instead, the vacuum leak occurred at the Rear Plug of the Fuel Injection Manifold!!  This steel plug was permanently installed in the alu Manifold at time of manufacture, and is not normally subjected to any maintenance [or suspicion!  One might not even spray carb cleaner in the area as the usual first method of trying to localize a suspected Vacuum leak!].  The plug is not unlike those found on the engine block Cooling System, and it was in-fact also installed by dimpling the convex shape, which expands the seating area and this sets it into place...and that should have been it! 

...but add 50 years of the two dissimilar metals in contact, possibly in an active atmosphere, and Luigi Galvani sends his greetings! Besides making a dead frog's legs twitch by applying electricity, and thus discovering "Animal Electricity", [and possibly giving Mary Shelley (and Mel Brooks ideas)], he also discovered that he could generate electricity by having two dissimilar metals in contact (hey, it's a Battery!) ...and the rest is history as they say...and the bane of many mechanics and electricians (and sailors, and many more...!), because the only wrinkle is, that one of those two metals will deteriorate!

The vacuum leak occurring at the Manifold Plug will likely be minor, but any uncontrolled air we can prevent from sneaking into the manifold is a good thing!   


Location of Plug on rear of FI Intake Manifold.


Vac Leak at Manifold, Occurrence 1:  

A poster to the British VOC Forum has confirmed a vacuum leak at this Plug. [This owner finally located the source of his pesky idle problems by charging the Intake with smoke, a valid process when all else fails!  See also: ]

My response to his posting: 

"I expect your finding is a somewhat unusual occurrence ...but it would definitely contribute to idle issues in the D-Jet...and compliments on your technique for locating it!

The (est.) 2" Blanking Plug [Correction:  1 5/8" (41.5mm)] is set into the alu Intake Man at time of manufacture, similarly to an expansion ("freeze") plug in the engine (center displacement of the dome form, to expand sealing surface)...if it was replaced, and not properly installed at that time, or if it has simply developed a leak due to galvanic corrosion of the dissimilar metals, I would first check to see if the installation looks normal and proper, and if it does look OK, and the leak is around the edge, almost any fuel compatible sealer from gasket gook to RTV, to JBW [JB Weld metal filled epoxy.  See also:  Product Reviews ] will do the job until next time Intake Man is off, and access is good for a proper repair (removal and resetting with sealer!)

If you want to remove the BP with Intake Man in place, I should think that by removing the fresh air snorkel and opening the throttle, a broomstick or similar can be used to impart impacts to the back of blanking plug to "undome" it and persuade it out of position to allow removal...a simple cleanup of surfaces and reinstallation and resetting (with sealer!) will then give a permanent repair."


Plug is a 1 5/8" (41.5mm [seems to be a standard size!]) OD, similar in style (not necessarily OD!) to the common "Freeze-Plug" as also found on the engine Cooling System, installed by setting with an impact to the unmistakable circular witness-mark is evident from this operation, which distorts the convex shape and dimples this down, which slightly expands the OD of the plug, securing it into place as well as sealing it [...or it should!].  Some gasket sealer couldn't hurt to assure this, read on...! 


Closeup of the Plug in a Manifold from the SW-EM motor pool.   Plenty of surface discoloration, even pitting is evident.  I won't know the state of the seal, but next time that Manifold is under vacuum, the reader can be certain I'll be checking that there are no a matter of fact, knowing now what I know, and as simple as rework is when the Manifold is off the car, I'll probably remove and reseal the plug preventatively, before that Manifold is even put back into service!  

Cast Alu manifold, Plug, and chemically active area of interface of dissimilar metals! 



Vac Leak at Manifold, Occurrence 2: 

This time, in Hawaii, where there is no shortage of nice weather (Great!), but also no shortage of salty ocean air (less great!). 

Elliot Lucas pix, permission requested. 

Innocent enough looking...lots of surface oxidation is evident, undoubtedly helped by the ocean breezes it is exposed to...


During his search, a thin plastic sheet is undeniably drawn in by the vacuum, confirming a leak!  Such a vac leak will not be huge, and let in large amounts of air, and the Manifold Pressure Sensor will actually compensate and add more fuel since the leak will result in a slightly reduced Manifold Vacuum in the same way a Throttle open slightly more would, but this leak is anything but consistent, and the air it allows in is clearly not under control of the Idle Air Screw.

Screenshot from his video.  Evidence of a vac leak is unmistakable!


Occurrence 3:  

Setting idle on a B20F D-Jetronic FI system - Page 3 - VCOA Forum - Volvo Club of America



Other incidents of Galvanic Corrosion:  Realworld Experiences with Electrical Corrosion and ACZP

Main SW-EM D-Jet page:  Bosch D-Jetronic Fuel Injection Notes Compilation


Reference Information: 

In searching for "Freeze-Plugs" both cup and dished are available in various sizes, including the 1 5/8" (41.5mm) I measure here.  Without actually removing one from an FI Manifold, I would expect them to be the dished style. One can see that the actual contact and sealing area of the dished style is merely the edge of the disc (est. 1/16"), so it wouldn't take much in the way of galvanic action to compromise what needs to be a good seal between the atmosphere outside, and the partial vacuum inside! 

If one were to remove the plug, after confirming a vacuum leak, or even preemptively, a clean-up and inspection of both edge of plug and ID sealing area of Manifold should be performed to help with the decision if a given plug should be reused or replaced, and reinstallation with gasket sealer is certainly recommended! 


From an e-mail exchange:

Jim P. wrote:

Ron, the plenum's rear opening is 1 19/32", 40.5mm.  A 1 5/8" expansion plug starts of at 1 41/64". It won't fit into the opening like one does in a freeze plug block hole.



Thanks for the dimensional observation...I'll amend the tech article to reflect that it's a metric plug when replacing...I just checked recently and knocking it out with a 1/2 X 3/4" stick through an open Throttle could be done with little issue, so that, and reuse (with sealant!) is what I'm going to recommend as the simplest process.


Knocking the Plug out from the Throttle end...during the process, assure the Throttle is NOT subjected to any mechanical insults! 





Mel Brooks and Gene Wilder also experiment with reanimation of dead tissue...Teri Garr as Inga helps!


External material is attributed.  This article is Copyright © 2023.  Ronald Kwas.   The term Volvo is used for reference only.  I have no affiliation with this company other than to try to keep its products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, or worshipped, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you donít, youíre just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment (possibly about Galvani's frog legs...). 


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