help me read a vacuum gauge

I'm trying to diagnose a misfire. I don't really know how to interpret a vacuum gauge though. See this video:

Vacuum gauge video

Tough to read the numbers, but the needle is fluctuating rapidly in the 15 to 19 range.

Is this normal? Thanks.

Hallo. At idle there is normally the most vacuum of 18-20 inHg
At WOT it is 0 inHg.
That's the only values I known about XJ's. :wave:

'92XJ

p.s.the needle is fluctuating rapidly ;; that's the reason, why they use a scope for these diagnoses.

It's been years since I had one hooked up. According to Haynes, though, that would be excessive fluctuation. You didn't hold the revs up long enough to see, but if it smooths out at higher RPM's, Haynes says it indicates worn valve guides. I'm going to hook up a gage I have on my 2000, as an economy monitor. I'll see if it fluctuates like that too. My gut feeling is that it is normal.

winterbeater said:

It's been years since I had one hooked up. According to Haynes, though, that would be excessive fluctuation. You didn't hold the revs up long enough to see, but if it smooths out at higher RPM's, Haynes says it indicates worn valve guides. I'm going to hook up a gage I have on my 2000, as an economy monitor. I'll see if it fluctuates like that too. My gut feeling is that it is normal.

Click to expand...

thanks, that would be a big help. I'll hook it up again and rev it more, see what happens. would that cause misfire code, rough idle?

First thing is to pull your spark plugs and read them. You must have done that already if your doing vacuum now. What did they look like? Tan to white, dark brown, one plug different than the others? How many miles? 4.0 is alwa ys the assumption? What year?

sorry forgot to post the specs:

4.6 stroker w/25k miles. Its had this rough idle thing for about a month. I've been beating my head for a while trying to figure it out, checking sensors, wiring, etc. I figured I go back to basics.

Spark plugs all looked the same generally, kind of pinkish to tan-ish on the ceramic part, no carbon build up.

After clearing them, usually get 3 CEL codes: random misfire and then usually cyl 2 and 5.

Still need more specs, year, electronics, Renix, OBD-I, II, etc?

I would check compression next. Sounds like a real compression problem, bad valve(s), seats, or head gasket, or a leaky injector took out the piston ring seals / cylinder.

Vacuum fluctuation that large at idle is not good, unless you have a big open leaking vacuum line near the MAP sensor? (still not good, but not as bad as other causes).

97 OBD2, AW4, 4x4

heres the full rundown of what's been done so far.

electronics replaced/swapped with known good:
-MAP
-IAC
-TPS
-cam position sensor
-ran with a different ECU
-upstream O2
-toothpick check of distributor timing at TDC

checked but not swapped:
-Intake Air Temp
-Coolant temp sensor

did not swap the crank position sensor

fuel system/ignition:
-fuel pressure ok (49psi)
-did the 7 second fill up the container fuel pump test
-new injectors
-noid light check ok
-spark at all 6 cylinders
-checked plug wire resistance
-replaced ignition coil
-replaced dist cap and rotor

mechanical:
-compression ok (170 psi)
-had a shop do a leak down test and confirm spark, compression, noid light
-did a coolant pressure test - ok
-measured camshaft lift at top of pushrod (with rockers off) to check if dead lobe - not sure if this is a vaild test, but all the measurements were right on (maybe 0.001 less) for my cam
-cleaned air filter

oh yeah, I also replaced the head gasket. I thought for sure it was that, and in my haste didnt bother to check lifters while the head was off (stupid).

That looks like something in the valvetrain (sticky/burnt/bent valve, lobe going flat, etc). Assuming that you've read the plugs, watching it run with the valve cover removed may shed further light on the cause.

doktor_x said:

watching it run with the valve cover removed may shed further light on the cause.

Click to expand...

never done this - does oil spray everywhere, or is it pretty contained?

Did you have a machine shop go over the head?

With real good compression (170 is great) like that, assuming the guides are OK and not leaking oil and thus not periodically failing to seal at idle, and assuming you had the valves checked at a machine shop before re-installing the head, I would got back to that vacuum problem. Try plugging all the vacuum lines on the intake except the MAP, and see if the idle is still rough? The vacuum at idle should be pretty steady.

Have you tried new plugs?

What plugs are you running?

Ecomike said:

Did you have a machine shop go over the head?

With real good compression (170 is great) like that, assuming the guides are OK and not leaking oil and thus not periodically failing to seal at idle, and assuming you had the valves checked at a machine shop before re-installing the head, I would got back to that vacuum problem. Try plugging all the vacuum lines on the intake except the MAP, and see if the idle is still rough? The vacuum at idle should be pretty steady.

Have you tried new plugs?

What plugs are you running?

Click to expand...

need to check on the plugs. When I did the stroker, the machine shop did a full valve job. Not sure the quality, since some of the valves are sunk a bit (with a straight edge, the valve tips are not at the same height - not much off, but still off)

I did the vacuum test with just the MAP hooked up, with other openings plugged, same result. Noticed when I pulled, say, the brake booster line, that the idle jumped, then settled back in with the same vacuum. I would have thought it would be lower?

Maybe a leak in the gasket?

Also sometimes it searches for idle speed when it is put in park after a drive - sometimes hunts around 1200 then usually settles back in to 650 -700. It still fluctuates randomly, about 100 rpm or so.

OK, I dug through my tools and found an excellent 4" diameter professional level vacuum gage. I hooked it up to my XJ 2000 4.0. It read very steady at about 16.5. It had at most 0.2 needle fluctuation.

I was very glad that I tested it though. I discovered a 3/8 hole in the vacuum line that runs from the valve cover to the intake manifold which the PO had wrapped long-defunct electrical tape around. The hole may have been caused by the hood release cable rubbing it. Cut the plastic line and repaired with a 1 1/2" long piece of fuel compatible rubber hose.

Anyway, as numerous people with vacuum testing experience already told you, you DO have a problem. Your needle bouncing around is definately not normal. By the way, only about 0.4 difference in hooking to the brake booster port than the Crankcase Ventilation Port. My IAC must be working good, as the idle raced a little with either hose disconnected, but settled down within 3 seconds.

I found my old notes (URL location). Here is the web site with the how to on vacuum gauge diagnostics.

So far it sounds like a valve problem in the head, but an engine miss is also listed as a possibilty (bad FI or spark problem, but you seem to have those covered already). You might try adding a quart of MMO, to see if it is a sticking valve or lifter. Sticking lifters can freed up sometimes by taking the engine over 3000 rpm, after warm up.

Have you tried pulling one plug wire, or injector wire at a time to make sure each cylinder is actually firing?

Anyway, there are some really cool tests listed here that differentiate a vacuum leak from a head, lifter, valve guide.... problem.

http://www.fordf150.net/howto/diagnoseengine.php

Thank you guys for the responses. So I ran the vac gauge at around 2000, see the video here:

Vacuum again

I basically opened up the throttle, held it, then let it snap back. At the end you can hear it hunting for idle. Anyway, thinking its worn valve guides, leaking intake. I'm going to recheck compression for head gasket, and run it w/o the valve cover as suggested before. Then I'll prolly end up pulling the head and bringing to a machine shop to be looked at.

Also some other vacuum interpretation sites that I found in my search:

http://www.therangerstation.com/Magazine/Summer2003/VacuumLeaks.htm

http://www.secondchancegarage.com/public/186.cfm

We need to make this thread an FAQ! Nice find on the extra links there. They have some added info for sure!