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High MAP Sensor Voltage - Low Vacuum At Idle

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High MAP Sensor Voltage - Low Vacuum At Idle

Old 04-16-2019, 06:27 PM
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High MAP Sensor Voltage - Low Vacuum At Idle

This High MAP Sensor Voltage - Low Vacuum At Idle is kicking my rear. I've been given possibilities such as a "slipped balancer, aftermarket cam or the potential for slack in the timing chain". Another member advised in a PM that "For the most part a longer duration cam. That will cause a higher manifold pressure at idle. As for your engine, there are any number of things that could be causing the issue. Bad MAP sensor, timing chain off a tooth, cam swap, are just a few." I've done everything mentioned in the service manual for Code 33 and the condition remains.

Can anyone post a novice friendly guide to troubleshooting low vac at idle?
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Old 04-16-2019, 07:15 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

Have you done a compression test ?
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Old 04-16-2019, 07:45 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

Originally Posted by Tuned Performance View Post
Have you done a compression test ?


You have to create it (vacuum) before you can lose it.
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Old 04-16-2019, 07:47 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

No, not to sure on how to use the compression tester kit. I'm trying to understand the instructions.
https://www.harborfreight.com/automo...-pc-62638.html

Disconnect battery, Remove all plugs, Disconnect ignition cap to coil wire, Screw fitting into #1 sp hole & crank engine? Repeat on all remaining cylinders? *If that's correct, how am I going to read the gauge while cranking the engine, the hose isn't long enough. How is this testing compression if all the air is coming out of the sp holes?
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Old 04-16-2019, 07:51 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

You can disconnect the power at the coil grey connector with pink and white wire.
hold the throttle open and crank the engine over.
the gauge will keep the psi reading you don’t have to see it while cranking.
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Old 04-16-2019, 07:52 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

You are testing each cylinder individually. Removing all the plugs at once just makes it easier to crank the engine over (no compression). So only the cylinder with the gauge will build pressure.

A helper is the best bet...someone to crank then engine while you read and record the values. If not, there are remote starter switches...jumper wires go to the starter, with a push-button switch at the end of them that you can press and hold to crank the engine.

Or as Tuned suggested...he's a faster typist.
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Old 04-16-2019, 08:44 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

Originally Posted by Tuned Performance View Post
You can disconnect the power at the coil grey connector with pink and white wire.
hold the throttle open and crank the engine over.
the gauge will keep the psi reading you donít have to see it while cranking.
Perfect! If you drank, I'd owe you a case of cold ones for all your help.

Originally Posted by DynoDave43 View Post
You are testing each cylinder individually. Removing all the plugs at once just makes it easier to crank the engine over (no compression). So only the cylinder with the gauge will build pressure.

A helper is the best bet...someone to crank then engine while you read and record the values. If not, there are remote starter switches...jumper wires go to the starter, with a push-button switch at the end of them that you can press and hold to crank the engine.

Or as Tuned suggested...he's a faster typist.
So I have a plan in order, I can remove sp#1 and connect the fitting and follow Tuned's instruction to obtain compression of that cylinder? Why doesn't the test advise completing with all plugs in place? Don't we want the real compression? I mean, maybe i'm overthinking it...or just don't understand.
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Old 04-16-2019, 08:50 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

The results would be the same with all the plugs out or one at a time. With all the plugs out there is less resistance on the starter.
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Old 04-18-2019, 05:06 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

Trying the compression test after work, but found some interesting bits online though from a person w/ a 89 camaro 305 tbi and with 2.73 gears just like my 92.

Complaint: (NEVERLETITDIE) When the timing was set at anything under 6' i couldn't even break the tires loose. at a stop... this is where i started thinking im abit radical on the cam.

Brand:COMP CamsProduct Line:COMP Cams Xtreme Energy Cam and Lifter KitsPart Type:Camshaft and Lifter KitsPart Number:CCA-CL08-501-8Cam Style:Hydraulic roller tappetBasic Operating RPM Range:1,200-5,200Intake Duration at 050 inch Lift:212Exhaust Duration at 050 inch Lift:218Duration at 050 inch Lift:212 int./218 exh.Advertised Intake Duration:264Advertised Exhaust Duration:269Advertised Duration:264 int./269 exh.Intake Valve Lift with Factory Rocker Arm Ratio:0.488 in.Exhaust Valve Lift with Factory Rocker Arm Ratio:0.495 in.Valve Lift with Factory Rocker Arm Ratio:0.488 int./0.495 exh. liftLobe Separation (degrees):112Intake Valve Lash:0.000 in.Exhaust Valve Lash:0.000 in.Grind Number:XR264HRComputer-Controlled Compatible:YesLifters Included:YesLifter Style:Hydraulic roller

Response: (macertheracer) Sounds like the cam timing is off, what timing gears did you use? if you used a set that has 3 differant keyways you could have used the wrong mark. If you have to advance the IGN timing you are trying to catch the cam

https://camaroforums.com/forum/82-92...se-help-37453/

Hmmmmmmm

Last edited by 92RS-HeritageEd; 04-18-2019 at 05:09 PM.
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Old 04-18-2019, 07:16 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

Originally Posted by Tuned Performance View Post
The results would be the same with all the plugs out or one at a time. With all the plugs out there is less resistance on the starter.
I have no idea what this means BUT this is w/ the fitting in place of sp#1 engine cranking for a 8-Count & that grey connector unhooked from the coil...


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Old 04-18-2019, 07:26 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

135 psi is a good reading, Cylinders shouldn’t very by much.
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Old 04-18-2019, 07:41 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

If compression looks good, I'm at a loss. I was also told in a PM I could try checking for an aftermarket cam by "Measure the lobe lift at one of the pushrods (remove rocker). Multiply by 1.5 for valve lift, compare to stock cam." and "Check to see if the valve guides have been cut down for more lift clearance." but that was like speaking french to a newborn.
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Old 04-18-2019, 07:47 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

You got 135 on all cylinders?
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Old 04-18-2019, 07:52 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

Oh no, only accessible option was #1.
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Old 04-18-2019, 07:59 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

You need to check them all.
most sbc will be in the 135 to 155 range on a compression test.. cylinders need to be within 10% of each other
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Old 04-19-2019, 05:53 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

Cranking compression varies with so many factors it's hard to keep them all straight.
Discharged battery--slow cranking speed will reduce indicated compression pressure.
High altitude will decrease indicated compression pressure.
Mis-timed camshaft, worn intake cam lobe, broken rings, burnt valves...they'll all affect the indicated pressure. THAT is why having all the cylinders showing about the same pressure is more important than the actual pressure developed (within reason.)

You need to test 'em all. And if the engine is cranking slower as you progress through the cylinders, you'll have to charge the battery.

A faulty gauge...well...that goes without saying. How many people test their compression gauge for accuracy? I use a brass "tee" and the compressed air system in my shop. Compare the compression tester gauge to another compression tester, and/or to the gauge on the compressed air tank.




IN GENERAL, on these older cars, I usually see "about" 150 psi on good cylinders. Sometimes less, rarely more. As said, 135--150 is pretty common even if I'd really prefer 165--170.

A cylinder will RUN at 80 psi. I'm not saying it'll run well.

Last edited by Schurkey; 04-19-2019 at 06:10 PM.
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Old 04-19-2019, 06:18 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

Rain all day, no garage, unable to complete test on remaining cylinders.

Side Note: Speaking of slow crank, what if you get slow crank w/ a confirmed charged battery? Sometimes I get a slow crank on cold start just like the battery's dead but it isn't, then once the car has been started/warmed if turned off the next start the crank seems a tad stronger from time to time.
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Old 04-20-2019, 06:00 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

Originally Posted by 92RS-HeritageEd View Post
Rain all day, no garage, unable to complete test on remaining cylinders.

Side Note: Speaking of slow crank, what if you get slow crank w/ a confirmed charged battery? Sometimes I get a slow crank on cold start just like the battery's dead but it isn't, then once the car has been started/warmed if turned off the next start the crank seems a tad stronger from time to time.
1. A battery can be charged, but still be defective.
2. Corroded connections at battery or at starter, including the ground side of the circuit
3. Corroded cables including the ground side
4. Faulty starter motor including solenoid
5. Faulty engine
6. Faulty accessories (alternator, A/C compressor, etc.)
7. Excessively thick oil

An excellent test to verify cables and connections:
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Old 04-20-2019, 07:42 PM
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Re: High MAP Sensor Voltage - Low Vacuum At Idle

Given iffy weather today was light, I replaced a valve cover gasket. plan on finishing that compression test tomorrow. I'll can check connections at the starter then. After a overnight charge on the battery the car did have a noticeably easier crank so i'll have to get it tested. I noticed vac built to like 15-16Hg once at operating temp, and map voltage was the lowest I've seen yet at 1.8v BUT I had that vac ball in the finder disconnected and line plugged. 15-16Hg is Late Ignition Timing but have no clue how this relates to map voltage. At 6*-8* advanced already I don't want to push it further.
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