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I have a 96 5.7L Vortec Chevy Tahoe which I bought a month ago.

I was checking the sensor values using an OBD2 connector and noticed that the MAP sensor is not producing good values.

I see 28 kPA / 0.977 Volts when idle and 27.7 kPa / 0.918 Volts when throttle half open.

Now what I am wondering:

  • What I understand is that the MAP sensor output should stay static when RPM is steady. Is this correct? What's the ideal behavior?
  • Could this be because of a vacuum leak?
  • Does anybody know if MAP sensor for this vehicle is 1 Bar, 2 Bar or 3 Bar type?

I can remove it and take a look but I have suspicions that the previous owners might put a wrong type of MAP sensor. Does this question even make sense?

UPDATE: After seeing comments about the need for more data and possible latency problems in the scan tool, I have managed to create a nice graph with more reasonable data.

enter image description here

Both charts are in the same time scale. I wanted to have 2 charts because otherwise lines go over each other too much.

With these charts, things look much better to me than I initially thought.

For example,

  • A: When throttle is at ~25%, engine load is at ~55% and engine speed is at ~2000 RPM, I can see the MAP at ~70 kPa.
  • B: 50% throttle, 100 kPa

But I don't understand C. When idling throttle position is %0, RPM is ~600 and the load is 25%. MAP sensor is at ~40 kPa which makes sense when I compare with engine load. This probably means the engine load is too high when idling, doesn't it?

Any suggestions to provide better data?

  • Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Apr 25 '18 at 13:08
  • 8 inHg is a little low. Whats the elevation and does the baro reading look normal? If you put a vacuum gauge on does it add up to atmospheric pressure? – Ben Apr 25 '18 at 15:46
  • You are best to take voltage readings directly at the map sensor before replacing it. – Moab Apr 25 '18 at 16:46
  • @All OK, do you have any symptoms, or just trying to figure out what seems to be wierd readings? Your MAP values from the only 2 frames provided is normal. The data update speed may not be quick enough to pick up an initial drop in vacuum, or rise in kpa when the throttle is 1st opened. Take a reading at about 60mph on a flat road and if things are not too far off, it should read in the 40-60 kpa range. – Milison Apr 26 '18 at 12:59
  • Most GM service manuals have a pretty ironclad method of testing the MAP with a multimeter. I don't have the procedure on hand for your car, but have a look around. – 3Dave May 3 '18 at 23:28
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+50

“Any suggestions to provide better data?”

OBDII data is just another raw data set and the most important step to an analysis is to define the question, or problem that you are trying to solve. This Quora post has a few decent answers that can also be applied to analyzing a vehicle’s OBDII data.

https://www.quora.com/What-are-the-top-5-things-you-should-ask-about-a-dataset

When it comes to engine performance diagnostics, understand that OBDII mode $01 is designed to give us hints toward drivability complaints whether it be perceived by the driver, or in the form of a MIL (Malfunction indicator light) on the dash. Think of mode $01 as a very limited birds eye view of only a portion of the possible data that could be collected.

As mechanics, we always start with the complaint. It is our job to come up with effective questions to ask the OBDII dataset. It might help guide us toward problematic systems that could be the root cause of the original complaint. For example, a rough idle when the engine is warmed up, and no codes stored. Once we can duplicate the problem, at this point, we start asking the datalog or dataset questions. One of the first things we ask is “What are the fuel trims doing during the rough idle?” They will either be normal, positive, or negative, steady or erratic. Depending on the answer, we then decide which systems integrity needs to be verified. Some questions might be answered in the datastream, but more than likely we will have to use other testing equipment in order to verify those systems. For example, an ignition scope, vacuum gauge, fuel pressure gauge, and compression gauge, to name a few. It would be nice if all engine faults came with an error traceback as with python code. We could then diagnose it down to the root cause without ever raising the hood, but what fun would it be if we didn’t get dirty from time to time?

“This probably means the engine load is too high when idling, doesn’t it?”

There are many factors that have an effect on manifold air pressure at idle. For instance it is common for the MAP to have a variance around 1 inHg or 3 kPa when idling in drive vs park. Also an increased electrical load, your elevation, and engine mechanical integrity are all a factor that contribute to your MAP reading. MAP readings from OBDII data cannot tell you what a vacuum gauge can. Does the vacuum gauge hold steady, flutter, or float up and down?

Looking at C on your graph, it looks like your map is sitting in the middle of 25 and 50 which puts it at about 37.5 kPa. According to an OBDII datalog taken from a known good 1998 Chevrolet k1500 5.7 liter, the average of 41 frames taken at a warm idle, in park, and all accessories off, is 35.76 kPA. (source - pidfusion.net) That equates to about a 0.5 inHg difference in vacuum, and that is nothing I would ever be concerned about.

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It appears to me your MAP sensor is bad, as the readings of the MAP does not mesh with what this chart I pulled from this website:

enter image description here

According to this chart, as vacuum increases, voltage should go down. Yours appears to be going in the other direction (vacuum goes down, so does voltage), as well as the voltage not being in the range which is expected as per the chart.

As for the map sensor, if this is not a boosted application (no turbo or supercharger), you only need a one bar MAP sensor. From your description, you aren't running anything which produces pressure, so can assume your MAP sensor is stock and only covers one bar.

As for whether the MAP should stay steady, it will for the most part, but variation will be present because even in a V8 the intake strokes are not happening all at the same time. There will be pulses present in the vacuum. As you get more cylinders in an engine (with a given valvetrain lift), the vacuum pulses should become less noticeable at a given RPM. I haven't measured it, but would assume at idle the vacuum reading will be very steady because this is the greatest amount of vacuum. Then, as you dip off of idle, the vacuum will be a little more erratic, but should become more steady as the RPM goes up because the vacuum pulses will overlap more as well as the ability of the sensor to register those changes will be less (more vacuum pulses per minute). Again, this is an assumption on my part. Overall, the vacuum pulses should be minor blips. If there are wild swings, then I'd suggest you may have some intake valve sealing issues.

PS: The stock MAP part number is: ACDELCO 213796

  • I think the chart is misleading. At idle map is lowest and vacuum is high, as the throttle opens map increases and vacuum decreases. The readings not squaring with the chart could be due to elevation or weather. – Ben Apr 25 '18 at 17:06
  • @Ben - I agree with your assessment of idle v. vacuum, but I'm not seeing as how the chart disagrees with either of our assessments? It shows right on the chart (top part), idle to be to the right side (high vacuum/low MAP voltage) and left to be WOT (low vacuum/high MAP voltage). The OP stated the MAP voltage was high at idle and went down as vacuum decreased. This is the opposite of what should really happen. I'm betting either a bad MAP sensor or the wrong part altogether. – Pᴀᴜʟsᴛᴇʀ2 Apr 25 '18 at 18:43
  • i must of misread the first paragraph under the table. Two single frames of data isn’t really enough to go on if OP provided a datalog (snap throttle and wot)it would help show the bigger picture. – Ben Apr 25 '18 at 19:06
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Before going through all the map sensor testing...first and second step suggestions for a code 33. 1. Make sure all your spark plug wires are actually connected 2. Replace Vacuum lines. 3. Trouble shoot map. I replaced all my plugs and wires and thought i connected the wire all the way but #1 cylinder wire popped loose from the plug. This ended up being what threw the 33 code. I started chasing the problem by replacing the vacuum lines. Only after doing so noticed the loose spark plug wire.

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