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Discussion Starter · #1 ·
Hello. I have a 1998 Nissan Sentra 1.6L it has a vacuum leak and I'm just wondering if someone can tell me if this hole in the intake manifold is supposed to be there?
Thank you in advance, any advice is appreciated.
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The hole looks like it's just the bottom end of the hole for the bolt shown above. When the engine is operating, do you feel any vacuum at the hole when you put your finger over it?

To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be around 21 InHg. If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
 

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Discussion Starter · #3 ·
The hole looks like it's just the bottom end of the hole for the bolt shown above. When the engine is operating, do you feel any vacuum at the hole when you put your finger over it?

To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be around 21 InHg. If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
Thank you for responding, I do not feel any vacuum coming from it. I'm just reaching trying to figure out what is going on. The car is progressively getting worse it tops out about 40 mph and stutters and jerks the whole time while accelerating. I hooked a vacuum gauge up to it and it was pretty steady at about 20. The other day I took it to the mechanic and he said the fuel filter was pretty bad so he put a new one on and also replaced a cracked hose but it still didn't fix the problem.
Yesterday I replaced the upper intake manifold and checked the hoses for cracks and stuff. The other day it was putting out a couple codes, one for the map/barrow switch, and one for cylinder three misfire.
I don't know a whole lot about cars, my dad was a mechanic when I was growing up and he taught me a little. And I just can't afford to take it to another mechanic. So your help is very much appreciated!
 

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When diagnosing an engine with rough running like you're encountering, there are certain things that must be done prior to replacing parts or performing service. If you have a 1981 or newer vehicle, it will most likely have an engine management computer or what's called an ECU. The computer monitors the functions of a number of engine compartment components. When a problem develops in a system that is monitored by the computer, codes which relate to the particular system will be set and retained in the computer’s memory. When this happens, the check engine light on the dash will illuminate. If the light stays on continuously this is called a hard fault and is the easiest problem to find because the problem is there right now. If the light comes on and goes out, the fault is intermittent, meaning the problem was there momentarily but has cleared up. Both these conditions will set codes that are retained by the computer for later investigation.

In most vehicles these diagnostic codes will remain in memory for fifty engine warmup cycles. If the problem does not reoccur, the codes will be erased by the computer. Steer clear of a mechanic or shop that replaces parts without doing the proper diagnostics first. Checking for computer codes first is important because it may lead you to the rough running problem. This is what you should expect a competent mechanic to do if you present them with this kind of problem. Perform an ECU code readout with a portable scan tool to see if any fault codes are set. The tool can be purchased at most auto parts stores. Post the actual codes here on the forum so that we may be able to help you further.

Several things that could cause misfires:
  • Is the misfire occurring on any particular cylinder? What brand of spark plugs are you using? You should be using OEM NGK plugs; other brands such as Champion or Bosch many times cause driveability problems in Nissan engines.
  • The camshaft position sensor (CMPS) may be marginal. A common problem with random engine shutdowns or rough running is a marginal CMPS. Most of the time when this happens, the "check engine light" never comes on; subsequently when performing an ECU code readout, there was no fault code set. The CMPS is located inside the ignition distributor and is not replaceable by itself; the entire distributor needs to be replaced.
  • Incorrect fuel pressure. Tee-in a temporary fuel pressure gauge between the fuel feed hose and the fuel rail. The readings at idle should be as follows:
  • with vacuum hose connected to the fuel pressure regulator: 34 psi
  • with vacuum hose disconnected from the fuel pressure regulator: 43 psi
* Check is your engine grounds for tightness and oxidation. With the car running, put a voltmeter on the lowest scale and measure from the block to the negative battery post. It should read no more than 50 millivolts (0.05V). If the ground is good, inspect the MAF for lint, butterfly wings, or any other kind of junk.
 
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