MAP and Vacuum Question

[#2875]

First of all I need to get out some background info. I just bought a SY and gave it a tune up, new FP and FPR. I have a stumble at low rpm and the truck seems to lack power slightly all over the rpm range. With the stock chip it is hardly noticable but there is a slight hesitation. With the ultimate chip the stumble is really bad. I don't know why my truck is stumbling any more than the rest of you do... and I have found A LOT of you with the same problem in my searches; probably with all different causes. All we can do is make guesses based on the symptoms and start replacing shit. One other thing I can tell you is that when I reset the ECM the problem gets better then gradually goes to hell again.

What I want to ask the forum is based on what I have found using turbo link. #1... When the truck stumbles I observe a lean reading from the o2 sensor FWIW; what might cause this?

#2... My MAP is around 12 at idle and vacuum is 13. I have a friend with a stock SY with 15K that I hooked up to and recorded his info for a constant and I pulled a guys spreadsheet from the web who ran a sub 13 sec pass. The only difference I noticed was that their MAPs were around 8 at idle and their vacuum readings at idle were around 23. What would be causing my MAP to be so high? Any additional info would also be appreciated. Thanks.

-matt

 

[sytyguy]

O2 readings can be skeewed by a whole number of reasons. Basically, if you have a sensor going south, causing the mixture to change, then the O2 readings will inherently change and the computer may "accidentally" blame the O2.

I've only seen MAP readings off due to 1 of 3 things:

- Bad MAP
- Wrong MAP
- Vacuum Leak

Fixing a bad MAP is easy, replace it with known good unit (borrow from fellow SyTyer to test).

A wrong MAP is also pretty easy to fix. Since you're hotswapping the stock and Ulitmate chip, I have to ask a question......are you changing the MAP sensor with each chip (2bar for stock and 3bar for Ultimate)?

A vacuum leak is also "somewhat" easy to fix. Either spray starting fluid around the vac lines while the engine is idling (you'll hear the engine rev when you hit a leak) or do the bike tube trick (search).

Rule these 3 out and we can go from there. HTHs and let us know what you find.

Hood

 

[#2875]

I have sprayed ether behind the manifold and everywhere else I know there are vacuum lines. I have (2) 3bar chips and my stock 2bar chip and I change them every time I swap the chip as well as change the fuel pressure... 50 psi with vacuum line removed for ultimate and 37 psi with vacuum line hooked up for stock.

 

[KBowman]

vacuum is 13. I have a friend with a stock SY with 15K that I hooked up to and recorded his info for a constant and I pulled a guys spreadsheet from the web who ran a sub 13 sec pass. The only difference I noticed was that their MAPs were around 8 at idle and their vacuum readings at idle were around 23

His sounds right yours no.
First: Use a mechanical gauge to verify the actual actual vacume at idle. If indeed it is that low (13) which I doubt because it would probably not idle at all, plus if it was a big external vacume leak you would hear it. Does it hold fuel pressure when you turn the key off?

Second: If the mechanical readings are normal for vacume at idle (20- 23 I would say) Then it will probanly be a map sensor or map sensor wiring.
Determine which of these situations you have first.
Hope it helps

 

[#2875]

mechanical gauge reads the same as value from OBD... about 18 at P/N idle. also, I just used up another can of starting fluid and found no leaks. I read about the bicycle tube trick but wouldn't the air leak out of the open valves? anyway, i don't think i have a vacuum leak... problem yes, engine compartment leak no.

 

[#2875]

oh and yes I am holding fuel pressure for a long while after shutdown. what are the symptoms of a bad egr solenoid? is there any check?

 

[KBowman]

oh and yes I am holding fuel pressure for a long while after shutdown. what are the symptoms of a bad egr solenoid? is there any check?
I thought you said it was 13, 18 is not nearly as bad. fuel pressure holding tells you that you do not have an injector stuck open causing a lower vacume reading, at least not when it is shutoff.
bad egr valve solonoid: eratic idle or hesition
Check: disconnect vacume line at egr valve and plug, egr will not open then. There probably are specific test for the valve controler solonoid and I can look in the book if you dont have one. But eliminating should test what you want to test.

 

[KBowman]

I did not mention but having the mech gauge the same as the obd means it is not map sensor related because it (map sensor) is reporting the right values for what the motor is seeing.
Low vacume can be caused by a lot of things and I am currently working on a simular problem on my truck. Most low vacume situations are caused by a mechanical problem. First check for external vacume leaks like you have and then check the compression of the motor. Yes compression test.

 

[#2875]

vacuum is 18 in p/n and drops to 13 when it goes into gear. i put on a new egr valve last night (no change) and i'm going to clean the IAC valve tonight. if that doesn't work i guess i'll drop in a small block... it would be much less work/money/bullshit in the long run; this engine has been nothing but a disappointment.

 

[KBowman]

Whats the altitude where you are located? You should be able to check to see from obd what the iac is doing.

 

[#2875]

600 ft. how will i know what the iac is doing?

 

[KBowman]

It should have a reading that tells you how far it is open. Mabey I did not explain myself but the iac just bypasses air passed the throttle plates. It is able to copensate for a vacume leak up to a certain degree by lessening the air that gets by the throttle plates. If your idle rpm is normal then it is compensating for it. I do not think the iac is your problem.
What is your idle rpm in neutral?
Low vacume is usally caused by a mechanical problem if the ign timing is right, whether it be an internal vacume leak (which could usally be seen as vacume at valve cover with the pvc out) or an external vacume leak (which you sound like have eliminated) or mechanical as far as a leaking valve, burnt piston,blown headgasket ect ect. Think of the engine as a big air compressor, it will pull a certain amount of vacume depending on how well everything is sealing inside of the pump.
Do you have a compression tester? Stick with basics and fix that or at least find out how tight the motor is. You may be trying to fix a hesitation problem but I would try and stick with the basics untill you have eliminated them. Hows the timing it can cause a vacume situation.

 

[KBowman]

Do you have data master?

 

[#2875]

i don't have a data master. i do have a product called turbolink. i do have a compression tester but i don't think the rings are bad or spun. the engine only has 50K on it and if the rings were worn that bad this early then there would be metal shavings in my filter, on the drainplug, or in the oil right? plus it would have a miss. if i was working on my fullsize with a small block i wouldn't hesitate to check the compression but since it takes an entire afternoon just to get to all the plugs in these stinkin things i have been reluctant to do that particular test. i did clean my iac valve last night and it seemed to do the trick. i have not had ample time to run the truck and every time i change a part such as this, map, o2, etc. things get better for a while and then go to shit but for now there is no hesitation. i don't mean to be an ass but i will question a $3 sensor that was made in china long before i'll tear in into a motor that is basically the same thing as the 265 small block that came out in 1955... minus 2 cylinders. too bad the $3 sensors cost us $75. [the following is not directed at anyone in this topic] BTW the iac valve can be taken out, cleaned, and put back in in about 15 minutes. i don't know why people take off their IC when all you need to remove is the egr and i'm not even sure you HAVE to do that. grind (or machine) a clearance groove in a 1-1/4 socket to make it look similar to a flare nut wrench. no problem.

 

[turbodog]

Getting the IAC counts from your scan tool will help diagnosis. If IAC counts is down near zero, this points to a vacuum leak (I know you sprayed starter fluid, but that is not always a conclusive test).

Low IAC can also indicate low fuel pressure. Do you have an adjustable FPR? What pressure are you running (FPR vacuum line disconnected and plugged)? Low fuel pressure could account for the stumble and higher severity with ultimate. Do you disconnect battery or pull fuse to reset BLM after each change?

 

[#2875]

fuel supply is kosher. i don't even have leave the cap on my fuel rail because i check the pressure so much. what else would cause this problem to be worse with the chip? at first i pulled the ecm and ecmb fuses. i don't think this works at all. i also tried disconnecting the battery neg. pretty sure this doesn't work either. the only thing that makes the engine run different is unplugging EVERYTHING from the ecm.

 

[#2875]

IAC is about 41 at idle. thanks. i never realized what that was. anyway is this normal? speaking of normal... my injector bpw is about 1.8 at idle and jumps to 30 every 10 seconds or so although after cleaning the iac valve it only jumps to 13 (at idle) although i'm not sure if it is related. is this a correction of some kind?

 

[turbodog]

I seem to recall the pulsewidth jumping is a GM software 'feature' that has been reported before. I have no personal knowledge about it. Your IAC counts are normal.

Another thing to check is timing. Find TDC on #1 and verify your harmonic balancer is OK (pointer points at 0 degrees) then set timing to zero with wire disconnected in passenger footwell.

This kind of problem has driven SyTy owners crazy in the past. Witness Paul (TechWeasel). I don't think he ever found his problem.

Best to assume nothing and methodically check everything (yes, including compression), making sure to use tests which are conclusive. The troubleshooting flowcharts in the manual are specifically designed to be conclusive. I find following them faithfully seems like the long route, but saves time in the end by avoiding mis-diagnosis.

Keep plugging, and good luck!

 

[#2875]

sounds nice... checking the timing i mean. i went out to the truck last month with a timing light and a screwdriver (my tdc checker). is the balancer pink? i wouldn't know because i can't even see the stupid thing. now how the hell am i supposed to check that?

 

[turbodog]

My balancer and timing indicator are visible behind the fan on driver side. My balancer was dirty and the timing marks were hard to see, so I jogged the starter and marked the line with white-out. There are 2 lines on the balancer, but if you use the TDC checker, you'll get the correct one.