concensus on vacuum reading at idle

[nynor]

i think i have a vacuum leak. my boost/vacuum gauge shows about 9, maybe 10 mm Hg. i am pretty sure it was more like 12 to 15 just last week. also, the AFM reads 16:1 at idle, i know that isn't what i want and that reading was more like 14.4:1 last week.

i am tearing into the intake system this morning.

any thoughts?

[skyhawk5421]

If this is a turbo we're talking about then that's definitely low. I can't remember exactly where but mine is always around 20 hot or cold. I'd check your gauge lines first as it's pretty hard to do a perfect job with that stock hard-as-a-rock KLR line

[Techno Duck]

Should be around 15ish cold and around 17-19 in-hg hot. Your AFR indicates a vacuum leak also, should be bouncing around 14.7afr at idle.. 16 is fairly lean.

Start with the easy stuff and check all the vacuum line connections. Then look at the hoses for the ISV and brake booster. Also check the barbed fittings on the intake manifold are not loose. Ive had two different intakes where the ports were loose, if this is the case; pull the fitting all the way out and clean everything as best you can. Then JBWeld the fitting back in. Then check intake manifold gaskets..etc.

[nynor]

what i've done so far:

i used some starter fluid while the engine is running, spraying all the fittings and around the intake/cylinder head interface. no change in idle.

i pulled all the major intake hoses to and from the turbo charger, etc. i tightened every fitting i could find. i did find two that were loose. i replaced a couple of the small vacuum line rubber fittings.

i inspected all the rubber boots very very closely and i found no cracks or holes. they look to be in pretty good shape, actually.

no change.

i'll double check the barbed fittings at the intake and the brake booster. i'll also double check the fittings to the boost gauge. however, with both gauges telling the same story, i am pretty sure that there is a pretty good vacuum leak.

i am thinking that the entire intake manifold is going to have to come off so that i can inspect the rat's nest and ISV.

one interesting thing is that i usually run the car without the DME talking to the O2 sensor, because it runs a bit richer and smoother, especially at startup. now, with the O2 sensor talking to the DME, it will idle at about 15:1 on the AFM and the vacuum creeps up to about 12. however, it will barely idle when it first starts, i assume that is the time interval for the O2 sensor to heat up.

[nynor]

one more thought:

the vacuum gauge may be reading low, due to the needle being a bit off "zero". maybe it is a fuel pump issue. checking the fuel pressure would be a LOT easier than tearing the entire intake system off, only to find that the problem is elsewhere.

the barbed fittings on the manifold check out ok. the bolts holding the manifold on are tight.

[nynor]

can a loose oil filler cap cause this kind of problem?

[nynor]

update: i took the manifold off, found six or seven loose hose clamps, hoses are good, i just changed them about three years ago. the small vacuum fittings are good. i did find one of the barb fittings on the manifold to be slightly loose, so i pulled it out and glued it back on with JB weld. i didn't find anything glaring besides the loose hose clamps, which i tightened/replaced. then i found that the oil filler cap was only about halfway on. the vacuum leak genesis coincides with the last time i topped off the oil. anyway, i had a minor misadventure, otherwise i'd have it back together and tested. i'll update tomorrow when i get it back together and fired up, weather permitting.

d.

[nynor]

update: yes, a loose oil filler cap can cause these problems. particularly, a high AFM reading and choppy idle.

thanks for the help, gents!