negative integer in vacuum field?

[89S10_Project]

Folks-

I am seeing an occasional negative integer in my datalog under the Vacuum field.

It seems to me that the 17 I am seeing in that field at idle indicates 17 pounds of vacuum. IE, the opposite of pressure.

Carrying that out to what seems a logical conclusion, when I floor it and I'm seeing -3 in that field... this would mean 3 pounds of pressure (in other words, above atmospheric pressure).

Just making sure I understand this properly before proceeding further.

Vehicle is an 89 S10 Blazer with a GenII smallblock, 5.7L TBI engine. I ran the numbers and it's a 94 crate engine, so it was clearly replaced in the 89 C1500 I pulled it out of.

EDIT: Correction! It's a Gen1 smallblock, 1994 crate engine, TBI 5.7, all else being as above. Thanks to Tom for educating me!

[1project2many]

Some MAP sensors will provide a signal to >100 kPa. Some ECM's will read this.

[89S10_Project]

Some MAP sensors will provide a signal to >100 kPa. Some ECM's will read this.

Please forgive my ignorance, but can you boil that down for me?

This is a naturally-aspirated engine. I seem to be seeing positive pressure in the crankcase. Should I be freaking out?

Engine runs great, except for a slightly stumbling idle I am still working on figuring out.

[Nasty-Z]

Use an actual vacuum gauge to verify , could be a sensor bad but I doubt it . I was looking at one of your earlier logs that you sent me and it didn't show a negative MAP value while I was looking at it . Might be another glitch in the 7747 data stream and the XDF file you are using to view it. Which .XDF are you using ?

MAP values generally start high (100) at high vacuum and decrease as engine vacuum goes down (0) Usually on a N/A engine you will not see true 0 MAP .

MAP values on high performance or altered engines can be useful to track down intake tract restrictions , such as with TBI engines .

One note : As for the engine , you have a Gen1 small block , not a Gen II , Gen II small block Chevrolet is the 92-96 LT1 and LT4 platform , Reverse flow cooling , gear driven water pump , Optijunk distributor , etc.... Might save some trouble down the road asking for parts as aside from them also being 350 CID there are vast differences.

TOM

[89S10_Project]

Tom-

Again, many thanks.

I swear, you are a font of information. I need to know half of what you've forgotten. :)

I've come to the conclusion that most of my sensors are of questionable age (other than TPS, temp, knock, oil pressure, O2, and now IAC, which I've replaced recently). Knowing that failures in electrical components can be intermittent related to temperature and many other factors, I have just decided to replace them all.

Yes, it's expensive, and may well be money wasted. I am saving the old ones for my next build, "just to get it running" sort of approach. Meanwhile, with this truck, at least I stil ldon't have a car payment!

Also, after convo with you and a few local guys, I am going to add an aftermarket (mechanical where applicable) gauge pod with engine temp, oil pressure, and vacuum gauges, as a "second opinion" for this sort of thing. Example: my Oil pressure on the factory sensor (also new, BTW)ranges from showing 60PSI to just about 0, and it's not temperature or RPM-dependant. I had 60 pounds when I installed the engine, and nothing about the engine's operation tells me I have an oiling problem- ie, no noise at WOT from the valvetrain or journals, etc).

Also- big thanks to you for correcting me on the typing of my engine. I don't know where I got the GenII thing from, thought it was when I looked up the block ID. IIRC, the Optispark ignition is at the front of the engine, right? This definintely isn't that, as the dizzy is where you'd expect it on an oldschool 350. Also, it's brand new as well, so there's a new ignition module and pickup coil to add to my list of new electrical bits.

One thing I am starting to ponder about this issue with my engine (intermittent stumble at idle and throughout the RPM range - most notable at idle though)... it only occurs when hot, near as I can tell. I have an Accel Supercoil bolted to the firewall, that has been in service in various vehicles (Fiero, Jeep, S10 since about 2002) since I bought it in 1995 or so. Yeah.. the coil is 21 years old... it might just be time for retirement. I'm going to get a stock replacement coil (which I understand is probably all I'll ever need- blame purchase of the Supercoil on being young and dumb), fabricate a bracket since I stupidly threw the old one away, and see what that does for me.

EDIT: also, to address your question- I don't recall off top of mind, what XDF I used. I agree it might be a glitch along the lines of the Injector Utilization and Spark Advance issues we've previously noted. Those are going to be my first forays into burning new chips- getting the patches applied so I know I have good data in Tunerpro.

[1project2many]

Please forgive my ignorance, but can you boil that down for me?

This is a naturally-aspirated engine. I seem to be seeing positive pressure in the crankcase. Should I be freaking out?

Engine runs great, except for a slightly stumbling idle I am still working on figuring out.

On some occasions you can get a 102-105 kPa reading from a "regular" MAP sensor. It is not usually a problem. If the MAP sensor is reading positive pressure it is most likely in the intake manifold, not the crankcase. If you are seeing positive pressure in the crankcase then you should ensure the PCV and breather hose to the valve cover are free of restrictions. If the values you are worried about seem to happen randomly and for only one or two frames without corresponding pressure increases and decreases, they are probably not accurate.

[89S10_Project]

Thanks, 1Project.

The MAP is fed from the TBI unit, so yes indeedy, is in the intake tract.

I am running the stock TBI unit for now, with an aftermarket K&N Filter. I am at some point going to replace this with the dual-horn cone filters, and there is a slight possibilty of boost in the future for this engine (10PSI at the absolute top end, more like 6PSI- my big boost fantasies stick around passenger cars, not vehicles with 3' tall tires). So I am just climbing the knowledge slope now, and you gents are super helpful.

As always, many thanks!

[lionelhutz]

The MAP value in a '7747 would be a 8-bit unsigned register which can go from 0-255. These values come from a simple A/D converter reading the 0V to 5V MAP output. So, it's not possible to represent anything above 1 bar or 100kPA in a '7747 ECM.

Now, errors in the sensor and analog circuitry could mean a MAP sensor needs a wee little bit of pressure to produce enough voltage to get to the register to reach 255, but the ECM would have no clue this was happening since it would believe 255 = 1 bar of pressure.

Looking at the adx file the MAP kPA is calculated from the raw datastream value and then vacuum is calculated from MAP kPA.

The vacuum formula is vacuum = 29.9 - (kPA * .349)

Solve for 0 vaccum (lowest possible WOT vacuum condition) -> 29.9 = kPA * .349 -> kPA = 85.7

It looks like an error in the adx file to me. Any time the MAP goes over 85.7kPA the vacuum will read negative. To me, the vacuum should only get to 0"Hg at 101.4kPA. The number in the formula should really be about 0.295.

I didn't do anything to verify the MAP calculation but if it produces an overly optimistic MAP number then that would also contribute to this erroneous negative vacuum you are seeing.

At the end of the day, don't forget that the data coming from the computer is usually raw values that mean little by themselves which is the converted to the different meters you're looking at in Tunerpro.