Engine vacuum results:problem indicated??

[slipnfall]

Hi there, I bought a MightyVac a little while back and have been finding myself using it quite a bit. Tonight I decided to check what readings my rebuilt DE was giving. I tee'd off the FPR, and made sure there wasn't a leak by shotting carb cleaner on the hoses while running.

Anyways, at idle I get a fairly stable 22Hg reading. I say fairly because on occasion(no timed interval), it will rise/fall about 1/2 a Hg. OK in my book. The problem I'm worried about is when I open the throttle. As soon as the idle begins to raise(to about 1100rpm, not much), the needle will drop sharply to 15Hg and sharply back up. There seems to be a *tiny* bit of lag in the rise of idle(like, 1/2second), but I don't know how fast the ECU is supposed to respond.

The MightyVac manual tells how to test for a leaking piston ring:-Vacuum will be low and steady(not the case here tho)-Raise idle to 2k-Close throttle quickly-Needly should jump 2-5" Hg above steady reading(mine drops)-Lesser gain indicates faulty rings, do compression test

When I do that, the Hg will rise when idling at 2k, to around 25Hg or so. When I let off the throttle, the needle will drop sharply to 15", like above, then quickly return to steady idle Hg.

Can anyone easy my worries? I have yet to get a compression gauge, this rebuild really has me drained.

FYI, the engine has maybe 50-60mi on it. Hasn't smoked a single bit, fluids are clean, no mixing that I can tell. I havn't taken it over 3500, or 50mph. Even then it was only for a minute at most, certainly no highway driving yet, just to/from work.

Also, the only known problem is my O2 sensor, it threw a code once or twice, for over/under signal condition. Timing was set w/a timing light. Completely stock, except the AIV is removed/capped, and the EGR disabled(I just capped the vacuum source control to the top of the EGR)

::edit:: Also there is an ever so slight hesitation on accelleration around 2k in 1st, mostly 2nd. Keeping on the gas will pull it through, it's hardly noticable(I originally thought this was just the feel of a different clutch, but after these results I'm wondering if the two are related). Stock air resonator are removed. ::edit::

Thanks everyone,-Jamie

[slipnfall]

Ok, I found out that the drop in vacuum is normal when opening the throttle. However I can't find an explaination as to why it is dropping in "Hg instead of rising when the throttle is released.

Heres the excerpt, from http://www.findarticles.com:"If the engine will run, attach a vacuum gauge to a vacuum source in the intake manifold. Start the engine and note the vacuum reading at idle. Now raise engine speed to 2500 rpm and hold it there. On a normal engine, vacuum should drop when you first open the throttle, then stabilize at a reading that's the same as or greater than the idle reading. If the reading starts to fall instead, suspect a blockage in the exhaust system.

The vacuum reading drops because the extra exhaust backs up into the cylinders at the end of the exhaust stroke. At the beginning of the next intake stroke, when the downward-traveling pistons should be raising engine vacuum, the extra exhaust already present in the cylinder lowers vacuum instead. A severe exhaust restriction will keep the engine from running at anything over idle rpm, because cylinder filling can't occur above that speed. "

[NISTECH]

ok first the slight hesitation at about 2k, because you say its so slight I think its likley because you dont have egr operation.

On the vaccum issue you have to take into consideration the Idle control valve. when doing this test you need to take idle control out of the loop. you do this by disconnecting the tps. when you go into decel the idle control valve cranks open till it sees a lower rpm then it adjusts to a stable point this is the reason you see a drop in vaccum. Even though you closed the big hole at the throttle plate you had another one open up in you idle contol. that test you conducting I suspect was written yrs ago when there was no computer control over idle.

[slipnfall]

Thanks 'TECH. I just wanted to eliminate anything that could possibly hose me up while I was getting this thing to run(thus disabling the EGR). One thing at a time. I didn't realize that this could actually cause hesitations(usually it's the other way around, when it gets clogged/stuck).

Anyways, I will re-test my vacuum today. I should also note that after I set my timing(TPS unplugged), I could only go so low on my idle adjust. The set screw is all the way in, but I can't get it to idle lower than 800 normally. Should be 700 w/TPS disconnected, and 750normally.

-Jamie

[NISTECH]

think about the egr thing for a second. if the egr is clogged it is not flowing, that would be the same as you intentionally blocking it off so it doesnt flow. make sence?

now you should be able to about kill the car when turn that screw all the way in with the tps disconnected. that screw your turning is its only source of airflow when the computer is out of the loop. you either have a vaccum leak or a stuck open iacv(idle control valve) or ficd(cold start valve)

[slipnfall]

Gotcha. Without the vacuum source on the EGR(from BPT), they remain closed via spring. I'm looking over the vac diagram now, and am getting more confused by the minute...I won't go into it now...

Anyways, I will check into these valves to make sure they are OK. The AC is removed, so that fast idle shouldn't be activating(that's the FICD BTW). Also the fast-idle for warm-up *seems* to be working: my idle will drop with warm-up. That leaves the one controlled by the ECU, the IACV-AAC(aux air contrl). Wish these things were more accessable!

Thanks again, I'll report back when I find out WTF is bleeding air in.

-Slip

[s13sr20chris]

just to fill you in i have seen cars that would pull just a hair more vac at engine speeds just over idle. that seems to be fairly typical.

[NISTECH]

chris it sounds as though hes more concerned with the 15in drop when the throttle place closes on decl, I am thinking thats the iacv opening to 100% until it gets to or near idle then it compensates to meet target idle.

[slipnfall]

Yah my vac reading seems to be on par at steady idle, I'm cool with that. The drop in vac does concern me, although I doubt it will help me pull any stronger to 2.5-3k. Above that the engine seems all mine. Funny because it was always pounded in my head that DE's excel in the lower end. So you think the EGR will help out a bit there? Logic would tell me that anything but clean cold air would be detramental(of course I'm betting the ECU alters fuel delivery when it thinks it's open, so this might be my problem).

I guess what's bothering me now is that something else is letting air into the loop, as you pointed out. I would not have thought twice about the idle screw, but now that I know it shouldn't be like that, it's gonna bug me untill it's fixed. :-)

Tomorrow I'm going to let it warm up sufficiently, then try squeezing the air intake tube to the 'cold start' valve. If the idle changes, I know that internal flap isn't 100% closed, and it's bumping my base idle up. Also the A/C valve: double check it isn't being activated. If those are OK I guess I'll have to try harder w/the carb cleaner to find a leak. I suspect I won't find any, if my vac readings are on par.

BTW another reason I disabled the EGR was because the short piece of replacement vac hose I used between the EGR and BPT was a bit kinked; I had forgotten about it untill the engine was already in(it's like a 90* bend, I'm going to get an elbow adaptor and make it right).

Best regards,-Jamie

[NISTECH]

if I am not mistaken there is only one air line coming off the intake tube that feeds both the ficd and iacv. via a steel tube that runs inbetween the intake manifold and plentium area. If you pinch that off at idle the car should die. I remember a 240 I had a vaccum leak on once was difficult to find but once I hit the sweet spot ,base of the egr valve gasket,with carb clean it made the idle change, but more importantly it made my o2 go full rich when I hit it. When I look for vaccum leaks I use the consult in line graph mode and moniter the o2(s) and watch for an increase on the graph, even if the leak is so small it wont effect the idle by spraying it you can see when you hit the spot ,it has an immediate reaction to the extra"fuel". This works well when trying to diagnose troublesome lean codes on the OBDII cars.

[slipnfall]

Well I'm tempted to give up on this idle issue... I just can't seem to find the problem. I really don't feel like tearing off those valves just to test them, they're such a PITA to get to.

I soaked *every* vac/air hose I could find with throttle body cleaner and nothing made a difference. I may have such a small leak that it won't make a difference in the sound, like you mentioned. However I don't think that this small of a leak would prevent my idle from dropping down lower(via the adjustment screw). I unplugged the A/C air valve, it made no difference.

Arghh...

I guess the last thing I could do is check the ECU for lean/rich condition. I know my O2 sensor needs replaced, so I don't know if this will be very reliable.

This is just more motivation to get my Consult board working. Well technically the board works, I can communicate with the ECU, but only in raw hexidecimal. I need to get cracking and bang out some code to go along with it, or break down and just buy the $20 program that's on the net(Conzult, SRTalk, can't remember the name).

Thanks NISTECH,-Jamie

[slipnfall]

Hey I hate to be a nag, but i'm still having trouble with this. I'm not so much worried about the dip in vacuum, or the high idle, as I am with getting my EGR functioning properly now. Tailpipe test is due soon(it's actually expired already) I have a feeling my BPT valve is bad, but I'm unsure. I can blow air through the top two ports with it out of the car, and applying a pressure to the bottom hose does not restrict the airflow through the top at all. Am I testing this right? I read in one of your older posts that you are/were an emissions specialist.

I can manually operate the EGR plunger, car stumbles like it should. With everything hooked up the way it should be, the car has a bad hesitation in 1st, 2nd. I'm hoping that getting everything working properly will also solve my minor/'slight' hesitation with it disabled.

FWIW, I did the O2 sensor test on the ECM, and it seems to be OK, however on accell the CEL comes on about 6-7 times per 10seconds, which would be rich, right? I'm waiting on a new OEM O2, should be here today: I got an O2 code from the ECU once or twice and figured I should just get one anyways, even if the original would get my by(it looks pretty darn old).

Thanks again,-Jamie

[NISTECH]

heres how the BPT works in its natrual state with no pressure applied to the bottom of it, it vents the vaccum to atmosphere through the outer ring. when pressure is applied to the bottom of the valve it closes the vent and directs the vaccum out the other port on top[to the egr]

[slipnfall]

Ahh, roger that. FWIW, the O2 sensor really helped to smooth out my accell. Nissan wants $50+ for the BPT, so it's junkyard time!

Thanks for your time,-Jamie

[NISTECH]

no prob . take a vaccum pump with you to test it before you buy it.

[slipnfall]

All's good. I hadn't hooked the EGR/BPT back up since the new O2 sensor, and it cured everything right up. I guess the older 02 couldn't 'keep up' with the EGR.

Passed emissions today, although it was not an exhaust 'sniff' test that I presumed.

Kudos,-Slip