At idle on my 420a, I get a reading of 13.3-14.2 in. Hg. Then at 3500rpm it's about 8.3-8.9, then around 4500rpm it's about 7.7-8.0. Does this sound normal, or should it be a little higher?

should be around 17 at idle.

Yeah, you should be idling at ~17-18... check for vacuum leaks!

hrm.. i need to check for vacumm leaks also damnit.!! when im at 3500rpms mine say 7psi }(

it varies by motor condition, temperature, throttle position, and engine load. More throttle will give you less vacuum. Go Wide open at 1000RPM, look, no vacuum. Concern yourself with vacuum at idle - if you're pulling less than 17inHG or so, i'd start looking at vacuum leaks and compression numbers.

My compression is 175psi accross the cylinders. Would this cause it to be around 13-14psi? Or you think there are leaks as well. And if there are leaks, where would the most likely place be? TB, or intake manifold gasket?

I'd say you have leaks. even with 175psi static compression, you should be pulling around 17-18inHG. Vacuum leaks are easy to find with a can of non-flamable brake cleaner. Spray short burts around the TB, intake manifold, injector seals, vacuum tees, etc.. when you hit a leak, the motor will sputter. Makes tracking them down a bit easier...

Thanks, I'll do that.
But, what does a low intake manifold pressure do to performance or how the engine runs? Anything I should notice if I got it up to 17in. HG? Rev up or down quicker, more power?

Anyone know how a lower intake manifold pressure will affect my car?

Also, does anyone know of something else I can use besides non-flamable brake cleaner to look for my leaks?

I've been trying to find any sort of leaks either at a gasket or on a hose, and I havn't come across 1 leak yet. If I get like 13in Hg, and it's supposed to be 17, there are some major leaks right? When I disconnect a large hose the intake manifold, the pressure hardly changes. It goes from the 13in. Hg to like 12in. Hg. Is there a chance maybe my MAP sensor is bad, reporting the wrong pressure?

Any tips?

what kind of vac gauge are you using? You *could* have a faulty one..Pick up a vacume tester at an autoparts store. They are very accurate, and are pretty cheap. Try that and see what you get. If you vac is that low at idle, the motor will prob not be running very well. How does the motor run?

I was using a pocketlogger to get my vaccum all along. But today, I hooked up a real vaccum gauge to my intake manifold just to figure out that my vaccume is 16-17 in. Hg. So it looks like my sensor is bad. Here's the interesting part...
When I rev it to 3500RPMs, my MAP sensor reads 8-9 in. Hg on the pocketlogger. But my manual gauge drops vaccum for a split second, then is steady around 19in. Hg. I think the manual guage is right, it's supposed to drop, then get more vacuum than it was at idle.
Looks like I have a bad MAP sensor? Where can I get one for cheap?
The engine runs great though...

I got a similar problem I can't figure out. My car will not idle normal, it stays around 2300rpm, unless you turn on the a/c then around 1000rpm. Anyway at 2300rpm my vac gauge stays around 15 or so, and with the a/c on (around 1000rpm) it says 8-9. I'm having hell finding a vaccum leak, any clue? I got a thread on it, tried alot of different things and yet to come up with a cure.

last time i looked on my autometer and greddy gauge. it was about 13-14 in hg at vacuum at idle. compression # was 190-195 on 881 je pistons, and last we checked no vacuum leaks. hooked up a air compressor with a bud at the turbo and blocked of the intake. any other ideas?
Michael
99gs
HRC Stage IV 8-Injector Setup
2GNT Member #32

But is my MAP sensor bad if its telling me very different vaccums than a manual gauge is? Could this be hurting the way my engine runs, and it should be replaced. If so, where can I get one?

If you have an SAFC, that's your cause - that's what an SAFC does.

for some reason every brake cleaner spray i got or find at a parts store say extremlllyyy flamableee.. so where can i get some non flame type to spray around?
Michael
99gs
HRC Stage IV 8-Injector Setup
2GNT Member #32

Ok,
Just got a new (used) MAP sensor and put it on my car. On my pocketlogger, I still get a reading of 13.3-14.2 in. Hg at idle. This is exactly what it said using my other MAP sensor.
When I hook a normal vacuum tester to my intake manifold, I get a reading of 16-17 in. Hg at idle. This is what everyone says it should be.
I don't understand why my logger would say 13-14 in. Hg at idle while it is really 16-17.
What is up here!?

Again, do you have an SAFC? Because if so, that is what is causing the discrepancy, because thet is what an AFC is supposed to do.

Nope, I don't have a SAFC.

Is 13-14in Hg is normal for the pocketlogger to read at idle for a 420a motor? User pn0ymahal said "last time i looked on my autometer and greddy gauge, it was about 13-14 in hg at vacuum at idle" He has a 99gs.

I know that I have no leaks, and my vacuum is actually 16-17in Hg right off my intake manifold at idle (using a manual gauge). So what causes some of our cars to see this as 13-14, rather than 16-17?

Any other people with the 420a (n/t) motor that can look up their intake manifold pressure at idle with thier pocketlogger, please post your results!

Then it is probably a misconfiguration in the way the logger reads the map sensor. Same thing happens with the TPS on almost all loggers out there. In a nutshell, your gaage is right and your logger is wrong.

Keep in mind that the boost gauge is reading "gage pressure" and the logger/MAP sensor is reading "absolute pressure". I explain the difference here:
http://www.2gnt.com/www/corbin/foolafc.html

So, at 0 psig on the boost gauge, the logger should say 29.9 inHG (absolute). And at 13 inHG on the boost gauge, the logger should say 16.9 inHG. Plus, you will have error in how the logger interprets the MAP output like Dino said.

Corbin

'95 ESI-T
HRC+FMIC+ETC...



Gimme fuel...Gimme fire...Gimme that which I desire

I'm getting that exact problem RIGHT NOW. I screwed with it some more, and tested from the port w/ no "T"'s and I get 17in at the intake manifold. My vacuum gauge reads 13 inside the car. My Autometer one I found to be a couple inches off. There is only a signal from the intake with a single "T" for my gauge inside and the BOV. Could my BOV be causing this "leak"?