99 civic rpm’s jumping at idle

Angelorod997

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I have a 99 civic , first codes I had was idle air control Valve, knock sensor, and o2 sensor, the car jumps rpm’s at idle , but when pressing on gas or driving it’s fine , even when started up before got it will idle at one speed , but after warms up it’ll start jumping from 1500 to 2500 , changed the o2 sensor , idle air control valve, tps sensor( calibrated it ) , changed map sensor , coolant temp sensor , bled the coolant system , gauged the valves , I’m lost at wht to do now, it’s still saying idle air control valve , knock sensor and now a cat code came up but I think that’s just from the fuel/ air mixture being off but I’m not sure what to do any ideas?
 

nd4sped

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Sounds like a fairly large vacuum leak somewhere. Common systems vacuum lines to check:
-fuel pressure regulator
-PCV valve
-Brake booster
 


nd4sped

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But if it was a vacuum leak wouldn’t it jump rpm’s right after starting?
Depends on the level of the vacuum leak.

I would recommend getting a vacuum gauge to measure the amount of vacuum the engine is creating. The gauge kit will come with a diagram giving you diagnostic information based off the amount of vacuum reading on the gauge.

 


Angelorod997

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Depends on the level of the vacuum leak.

I would recommend getting a vacuum gauge to measure the amount of vacuum the engine is creating. The gauge kit will come with a diagram giving you diagnostic information based off the amount of vacuum reading on the gauge.

I just found out the engine has been swapped with the same motor but not the ecu, the ecu is from the original motor does it need to be tuned?
 

nd4sped

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I just found out the engine has been swapped with the same motor but not the ecu, the ecu is from the original motor does it need to be tuned?
As long as the ECU is good, you will be fine.
 

Angelorod997

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As long as the ECU is good, you will be fine.
I’ve checked all the connections with a multimeter and they read what there supposed to , and if I buy a vacuum test where do I plug it into? Just a vacuum close to the intake? Does it need to be before or after something specific?
 

Angelorod997

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Depends on the level of the vacuum leak.

I would recommend getting a vacuum gauge to measure the amount of vacuum the engine is creating. The gauge kit will come with a diagram giving you diagnostic information based off the amount of vacuum reading on the gauge.

Just bought a vacuum gauge put it on a line that was close to the intake let it run and instantly got 20 vacuum pressure , after warmed up the car started to jump idle and the pressure went up to 21 . And it stayed constantly there, pushed the gas to about 3k Rpm’s went down to zero jumped back to 21 I guess next is to check the fuel pressure
 

nd4sped

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I’ve checked all the connections with a multimeter and they read what there supposed to , and if I buy a vacuum test where do I plug it into? Just a vacuum close to the intake? Does it need to be before or after something specific?
Your vacuum gauge should have come with a diagnostic sheet. Otherwise the HSM manual (available on my website for free) will have the correct info you need for the specific motor.
 

Angelorod997

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Your vacuum gauge should have come with a diagnostic sheet. Otherwise the HSM manual (available on my website for free) will have the correct info you need for the specific motor.
It did come with one , the vacuum pressure was what it’s supposed to be at , it says a normal motor should be at 17 - 22 pressure of mercury, my car started at 20 got hot and only went up to 21 hg , revved the motor to 3k rpm’s like it said went to zero jumped back to 21 on the pressure of mercury , it all checks out fine I’m going to do the fuel lines next
 

nd4sped

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Hg = Mercery
Readouts should be in Inches of Hg ;)

Proper feedback would be, motor creates and maintains 20 InHg @ idle and 3000 RPM. Vacuum stays constant.
Remember Vacuum is Negative "Pressure" and Pressure is positive. Just semantics ;)

Anyhow, nice work and your diagnosis. Sounds like you are on the right path now.
 

Angelorod997

New Member
Hg = Mercery
Readouts should be in Inches of Hg ;)

Proper feedback would be, motor creates and maintains 20 InHg @ idle and 3000 RPM. Vacuum stays constant.
Remember Vacuum is Negative "Pressure" and Pressure is positive. Just semantics ;)

Anyhow, nice work and your diagnosis. Sounds like you are on the right path now.
Yes this has been more of a headache then anything , hopefully it is the fuel pressure regulator or fuel pump fingers crossed .
 

nd4sped

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There should be two lines attached to the fuel pressure regulator.

  1. Small vacuum line attached to L shaped nipple.
  2. Larger Fuel line attached to bottom of FPR that is the return turn line that returning unused fuel to the tank that does not go to the fuel rail.
The fuel pump only pumps at one speed. this requires all Honda's before 2001 to have a return fuel line. Newer models have the FPR built into or attached with the fuel pump and typically only come as a complete assembly for replacement. This is NOT the case for your car thankfully.

If the FPR on your car has a vacuum leak, it is simply a vacuum controlled diaphragm (analog solenoid). The more vacuum applied to the FPR results in high fuel pressure. If the diaphragm has a hole in it, it will allow unmetered atmospheric air to bypass the diaphragm thus preventing the diaphragm from opening the valve. This results when the car is under throttle, not so much at idle.

Now if the diaphragm is somehow stuck "open" by any degree it can cause idle to be terrible as the engine should be running much richer than normal causing a rich misfire. Your spark plugs should show this as a result with heavy soot deposits.
 
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