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RESOLVED: 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?
Last edited by 92RS-HeritageEd; 05-20-2019 at 07:46 PM.
Reason: Resolved
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?
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.
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.
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.
Perfect! If you drank, I'd owe you a case of cold ones for all your help.
Originally Posted by DynoDave43
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.
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
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...
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.
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.
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.
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:
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.
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.
Can a person leave a charger on while testing, or only charge as cranking slows?
Originally Posted by Tuned Performance
Have you checked compression on all the cylinders yet ?
I started the test over, Cyl # 1 still at 135psi. Moved onto Cyl # 3 and this HF tool is making it more a PITA (stiff non flexible hose) but since I work from home i'll take another shot on lunch!
Update: Test postponed again. Battery low, cranking was slow and got slower (Cyl # 3 reached 65psi) and dash lights were very dim. I just charged it to full maybe 2 days ago, and the car ran for 30+ minutes or so. The alternator is a part that tested good at the parts store before installation so I have to get this battery tested since it was purchased used
Update 2: Ran to Advance Auto Parts and had the battery tested. Result was replace battery. Voltage measured at 12.35, CCA was 157 of the rated 800 at a temp of 70*F. To make matters worse, that's the one Pick-N-Pull rcpt I can't find!
Last edited by 92RS-HeritageEd; 05-10-2019 at 06:43 PM.
Can a person leave a charger on while testing, or only charge as cranking slows?
Go ahead and leave the charger in place, if the charger is robust enough to survive throwing current into a 9--10 volt load when the starter is cranking.
Years ago, this wouldn't be an issue--I'd just assume the charger was built properly. Today? I don't trust cheap-junk battery chargers. A larger battery charger is better.
Originally Posted by 92RS-HeritageEd
I started the test over, Cyl # 1 still at 135psi. Moved onto Cyl # 3 and this HF tool is making it more a PITA (stiff non flexible hose) but since I work from home i'll take another shot on lunch!
Another in a long line of reasons to stay the hell away from Hazzard Fraught.
Back to troubleshooting from this perspective since the Timing Chain Replacement had no impact on MAP voltage although the car ran smoother.
Engine ran smoother, but MAP voltage stayed the same? I would NOT expect that.
I'd be looking into the condition of the vacuum hose connecting the MAP sensor to vacuum, the condition of the vacuum port on the engine (throttle body?) and the condition of the MAP sensor and sensor wire harness.
DOES THE COMPUTER DATA STREAM SHOW THE SAME VACUUM LEVEL AS A MECHANICAL VACUUM GAUGE teed into the same vacuum port???
Engine ran smoother, but MAP voltage stayed the same? I would NOT expect that.
I'd be looking into the condition of the vacuum hose connecting the MAP sensor to vacuum, the condition of the vacuum port on the engine (throttle body?) and the condition of the MAP sensor and sensor wire harness.
DOES THE COMPUTER DATA STREAM SHOW THE SAME VACUUM LEVEL AS A MECHANICAL VACUUM GAUGE teed into the same vacuum port???
- Condition of the vacuum hose connecting the MAP sensor to vacuum? Will buy a new hose while getting the battery tested today.
- Condition of the vacuum port on the engine (throttle body?) How can I check this? I may have saw some plumbers tape on the exposed threads to that port where it screws in right behind the TBI.
- Condition of the MAP sensor and sensor wire harness? I checked this 1st, swapped out 3-4 spare MAP sensors with a random vac hose, issue remained. Followed steps in the gmsm to eliminate the ECM/Wiring up to the sensor in another thread. I get 5V from the power pin & the test light lights when probing the ground pin.
DOES THE COMPUTER DATA STREAM SHOW THE SAME VACUUM LEVEL AS A MECHANICAL VACUUM GAUGE teed into the same vacuum port??? Compared my most recent datalog against a video I took at some point while logging that day and yes they look in line. I'll double check this next time I have the car started.
Out of desperation 1st, and in close 2nd economical value I ran and grabbed a $50 Wal-Mart battery. Off the shelf straight to installation and boom not only was cranking the engine over 100X easier, Cylinder # 3 was also at 135psi after an 8 count. I'll be continuing the compression test after work but if this trend continues and all cylinders are in the + or - 10% range of 135psi what's next?
Run the engine, see if the MAP readings are still goofy. Lotsa problems created by low system voltage.
Ran engine (for kicks I added some Marvel Mystery Oil to both crankcase and fuel + periodic spritz of intake cleaner) 30+ min later the vac gauge starting holding steady 17-20Hg and MAP Voltage was the lowest i've seen. In idle Map Voltage dropped to 1V-1.25V or 1.25-1.75V w/ cooling fan on.
I've attached the datalog just in case anyone can chime in on why the MAP Voltage spikes to original high readings over 2V when placed in gear .
And... that victory was short lived. Went back out and tried to start it, weak crank but this time I heard what I can only describe as a "whining" sound while the engine attempted to slowly rotate. Battery is NEW (Currently @ 12.8V per DMM), that only leaves the starter and alternator. What is this whining sound? if it isn't one setback it's another!
Last edited by 92RS-HeritageEd; 05-11-2019 at 07:48 PM.
bad connections? maybe starter is on its way out. you can replace with a unit from the vortec era trucks depending on your flywheel diametre. pg260 you're running voltage @ battwry should be 13.5Vdc minimum, i like to see between 14 and 15 myself.
bad connections? maybe starter is on its way out. you can replace with a unit from the vortec era trucks depending on your flywheel diametre. pg260 you're running voltage @ battwry should be 13.5Vdc minimum, i like to see between 14 and 15 myself.
I find it strange that after the car sat all night the car cranked and fired on the first try (battery voltage went from 12.8 to 12.5). Do starters act woozy when they're on their last leg? like Slow cranking when hot? I checked the connections at the starter I could see and it's tight. The cars running right now in fact.
SideNote: Checked Battery Voltage w/ DMM and it shows 14.4V while running.
Last edited by 92RS-HeritageEd; 05-12-2019 at 10:37 AM.
I find it strange that after the car sat all night the car cranked and fired on the first try (battery voltage went from 12.8 to 12.5). Do starters act woozy when they're on their last leg? like Slow cranking when hot? I checked the connections at the starter I could see and it's tight. The cars running right now in fact.
SideNote: Checked Battery Voltage w/ DMM and it shows 14.4V while running.
Starters usually do crank slower than normal when hot and on their deathbed.
If you still have a 153 tooth flywheel on that engine I would recomeend a starter for a 90s LT1 F-car as a replacement.
Good news, it wasn't the starter or battery. Just a few random questions for anyone who wants to chime in...
-Why would it run like crap on cold start and get better as temps increase?
-Why once warmed and MAP voltage within spec it go to crap when placed in gear?
-The Vacuum Gauge teeters back to the LATEING Timing section, I've replaced the timing chain, set base timing to 0* & 4* what else has a role in (PLEASE don't say the ICM) Late Ignition Timing?
-What's the function of that Vacuum Ball in the driver-side finder?
Just a few random questions for anyone who wants to chime in...
-Why would it run like crap on cold start and get better as temps increase?
-Why once warmed and MAP voltage within spec it go to crap when placed in gear?
-The Vacuum Gauge teeters back to the LATEING Timing section, I've replaced the timing chain, set base timing to 0* & 4* what else has a role in (PLEASE don't say the ICM) Late Ignition Timing?
-What's the function of that Vacuum Ball in the driver-side finder?
Most of your questions have more than one answer. It's easier to answer the questions in reverse order.
Vacuum ball = vacuum storage so the heating system doesn't go to defrost and the cruise control holds speed when you climb a steep hill. At least, that's what it's for on my Lumina.
Vacuum gauge reads vacuum. It does not read timing. Just because the gauge face has some nonsense about "late timing" printed on it, does not make that the only cause of low vacuum. However...did you ever verify the timing mark on the damper actually aligns with "0" on the timing pointer at TDC #1?
There's a dozen reasons it might run good "cold" and run worse "as temps increase". First off, is this a matter of running good cold, or running good when in open loop. Not the same thing--but close.
Once warm, it idles OK, but runs bad in gear--another dozen reasons.
I have every expectation that the answer(s) are in the data stream--although I'm beginning to suspect the ignition module.
Most of your questions have more than one answer. It's easier to answer the questions in reverse order.
Vacuum ball = vacuum storage so the heating system doesn't go to defrost and the cruise control holds speed when you climb a steep hill. At least, that's what it's for on my Lumina.
Vacuum gauge reads vacuum. It does not read timing. Just because the gauge face has some nonsense about "late timing" printed on it, does not make that the only cause of low vacuum. However...did you ever verify the timing mark on the damper actually aligns with "0" on the timing pointer at TDC #1?
There's a dozen reasons it might run good "cold" and run worse "as temps increase". First off, is this a matter of running good cold, or running good when in open loop. Not the same thing--but close.
Once warm, it idles OK, but runs bad in gear--another dozen reasons.
I have every expectation that the answer(s) are in the data stream--although I'm beginning to suspect the ignition module.
What was it? #1 suspect, broken wire on the 2-wire connector to the alt.
Did you ever verify the timing mark on the damper actually aligns with "0" on the timing pointer at TDC #1? Verified in timing chain replacement.
Is this a matter of running good cold, or running good when in open loop? Runs like crap physically cold, rarely enters closed loop. As I watch the data in TunerPro the MAP Voltage comes down and engine runs smoother as temps exceed 125*. UPDATE: It's the strangest thing EVER. Car below 125* runs like a POS. Reached 125* and this time it ran like a DIFFERENT car, MAP voltage was good, and the Vac Gauge remained in the green 17in - 20in but RPM's were in the 1K-1.2K range.
I'm beginning to suspect the ignition module. Are there any tech articles on what exactly the ICM does, how it advances or retards timing based on RPM or engine load? Are those parameters stored on the ICM itself or in the PROM? Can any ICM from any GM car with a distributor be interchanged, is the spark output different?
Last edited by 92RS-HeritageEd; 05-18-2019 at 11:54 AM.
Reason: NEW info
What was it? #1 suspect, broken wire on the 2-wire connector to the alt.
Might affect battery charging. Won't affect starter if the battery is charged.
Originally Posted by 92RS-HeritageEd
Is this a matter of running good cold, or running good when in open loop? Runs like crap physically cold, rarely enters closed loop. As I watch the data in TunerPro the MAP Voltage comes down and engine runs smoother as temps exceed 125*. UPDATE: It's the strangest thing EVER. Car below 125* runs like a POS. Reached 125* and this time it ran like a DIFFERENT car, MAP voltage was good, and the Vac Gauge remained in the green 17in - 20in but RPM's were in the 1K-1.2K range.
Runs better above 125 degrees, but won't enter closed loop?
Originally Posted by 92RS-HeritageEd
I'm beginning to suspect the ignition module. Are there any tech articles on what exactly the ICM does, how it advances or retards timing based on RPM or engine load? Are those parameters stored on the ICM itself or in the PROM? Can any ICM from any GM car with a distributor be interchanged, is the spark output different?
The first distinction among GM ignition modules is the number of connections. GM started with 4-pin modules and zero timing control. Then a couple of 5-pin modules that could retard timing based on some external electronics--a knock sensor, for example. Full computer control of timing came with a seven-pin module. GM went to an eight-pin module later. You almost certainly have the eight-pin module--but it wouldn't hurt to verify that. There's a lot of superstition about ignition modules, even when they're all Delco-branded and supposedly high-quality. I read--on this very web site--about "Latency" timing variations among various part numbers of eight-pin modules. On one hand, the guy presenting the info seems to have done considerable research. OTOH, I'm VERY suspicious of his results, because supposedly "different' modules with considerable influence on ignition timing are considered interchangeable by GM--and if the module really affected the timing, there'd be EPA-compliance issues that would have to be resolved before the part could be certified as a replacement.
Show up at any parts-store counter in any parts store in the USA or Canada, ask them for an ignition module for your year, make, and model. They probably have one in stock. They probably have TWO in stock--a "cheapie" and a good one. If the "good" one is a Delco, Standard, or Echliin brand, you're good-to-go. If their "good" unit is some off-brand, find another store.
04-16-2019, 06:27 PM
the MAP Voltage spikes to original high readings over 2V when placed in gear . 
What is this whining sound? if it isn't one setback it's another! 
