I was under my hood yesterday and I discovered that a few of the 4 middle bolts that secure the upper inlet manifold were a little less than 22 NM or a lot loose. One or two (can't remember now) were so loose I could unscrew them with my fingers. (The 2 bolts at either end were secure).

I torqued the loose bolts back to 22 NM.

Prior to my discovery, there were no perceivable problems. The key word here is "perceivable". As in "perceivable by me when the engine is running and/or I'm driving".

Could the finger loose bolt create a vacuum leak isolated and small enough to only affect one bank of the upstream O2 sensors? Making that O2 sensor give erratic readings and/or lean readings? The further impact being to fuel trim ratios, fuel injection etc?

What does a vacuum leak sound like? I'm inexperienced enough that I suppose it's possible if the noise was subtle or intermittent enough that I wouldn't recognize it if I heard it. BTW, I tested for vacuum leaks prior using a spray bottle of soapy water and found none (either because of operator error or it wasn't there when I tested).

Besides a vacuum leak, what kind of impact could/would these loose bolts have on the engines performance and problems etc?
Anything else?

Should I replace these bolts? or use a heat tolerant thread lock goop?

Thanks,

Thomas

 

I had a couple(over half) loose lower intake bolts a couple years ago. It caused a vacuum leak large enough to set off the CEL. It kept throwing either a 1176 or 1178 code(cant remember) which was a general lean condition code. After replacing the pre-cat o2's, cleaning the MAF sensor, plugs, wires, fuel filter, etc,etc. It was when I went to replace the valley pan gasket because of the vacuum leak did I realize they were that loose.

So, after a too long un-needed story of mine, no, I don't think it would cause a problem. Just to be safe, you could run some intake cleaner through it. Vacuum leaks tend to cause carbon build-up. A big enough vacuum leak sounds like an air-sucking sound. Mine did not have that, that I could hear.

 

Absolutely. I think you're thinking the right things here.


You may not even be able to hear a leak like that. Some vacuum leaks I've hear sound like loud whistles or sick ducks (one of the larger vacuum lines on my pickup that split and was vibrating against itself with full idle vacuum), some just a hiss. Some not at all.

One of the standard ways to detect them is with a propane torch (off, just flowing propane). Go around the areas that may be leaking and see if your idle changes. You can do the same with starting fluid or carb cleaner (but the carb cleaner makes it run rough, unlike the starting fluid or propane).


Soapy water probably isn't going to tell you much. Soapy water is for pressure leaks (because the soap bubbles). At best, enough soapy water in a large vacuum leak might make it run poorly for a bit.

Long term? None other than fouled plugs and maybe a fouled O2 sensor. It would have to be pretty bad for a pretty long time to do damage to a cat.

Take one out completely. Does it look OK? If so, put it back in and torque it properly. Make sure it's nice and clean. The book doesn't say to use thread locker, and I'm not aware of this as a really common issue. I'm pretty shy about putting thread locker on things unless I'm really really positive it needs it.

If you were to use thread locker, I wouldn't worry about high temp. It doesn't (shouldn't) get that hot there. Blue would be sufficient. If you used red, I'm not sure how you would be able to get them hot enough to release the thread locker later on if you needed to remove them.