Rapid decrease in RPMs

[cdgator]

I have a 98 protege. It's a standard trans. This happens mostly at night for some reason but when i shift into neutral the rpms will come down very very fast then go very low to 500 or lower then come up and settle. Then usually a i will hear an odd noise which i am guessing is restricted fuel trying to go through the system and struggling due to the low rpms. If any one can point me at something to look for or has any ideas they would be greatly appreciated. This car is my DD so i really need to figure out what's wrong with it.

ps i am a newer member so sorry if i didn't give a good enough desc. or went about posting wrong I tryed searching for a similar post and had no luck.

[3ni_tricksta]

egr valve?

[bloodyspirit058]

im curious as to what this "resricted fuel" sound actually sounds like...

how many miles on it? last tune up? (plugs, cap, rotor, wires, oil, pcv ect.)

you have a vaccum gauge? might be able to do some quick tests to help figure it out. also is it fully warm, just started... temp where your at when it kinda does it? basically any info you can possibly think of.

[cdgator]

>the sound almost sounds like when someone drinks the last of a drink through a straw when just the little bit of liquid is being sucked through.

>it has around 2033xx miles. I bought the car about 2 months ago tops. I don't know when the last real tune up was. The owner before me didn't seem to take care of the car. I have personally put a new battery, plugs, belts, and thermostat in the car.

>i don't have a vacuum gauge but i don't hear any lines hissing. maybe i'm just not hearing them. If it asks up then it's when i start the car and continues the whole time i'm driving even when warmed up it doesn't always drive like that though.

>if you have any more questions i'll answer them i can't think of any other variables to add

i hope this helps you help me

[bloodyspirit058]

any codes when it starts acting up? also, when you start it up and it does this, do you notice the fuel pump kick on before you start cranking? (its pretty noticable)

can't think of anything else atm

[cdgator]

i don't know what codes you mean sorry and the car seems to start normal no odd noises or jumps by the car so i don't think so

i don't have any leaks btw if that helps at all

[bloodyspirit058]

codes would be the check engine light being on, if it is running bad enough it should pollute more and throw a code

[cdgator]

no lights or anything come on

[smokstac]

It's possible the light is missing or burned out. I went to look at a 97 several weeks ago and it ran like crap. The CEL light was burned out/removed and I picked up on the codes by having my laptop and USB OBDII cable. I walked away from it because the owner was misleading.

Take it to AutoZone or somewhere like that...they will scan it for free and it will tell you if there are codes being thrown that you don't know about. If it's running poorly the ECU should throw a code.

[cdgator]

today i got an oil change and they did a full throttle body cleaning and i seems to run much better i put a new wireset on today also next i'm going to replace the cap and rotor, and fuel filter. It idles i tiny tiny bit weird but the rpms seem to slow down around 1500 now then genlty come down if i stay in nuetral i'm still gonna see if it'll spit out any codes at AZ
thanks for the help another question now that this seems to be fixed what all should i do to try to tune up the car since the old owner did nothing.
>already have>
plugs
wireset
battery
intake clean
air filter
thermostat

[cdgator]

i went to autozone today and the 98 spit out a code. It needs a new map sensor. Where is the map sensor even at. If i ordered the part would i be able to put it in myself.
Help needed asap

thanks all

[smokstac]

To explain better:

A MAP sensor normally measures intake manifold pressure...low pressure means you have the throttle closed and high pressure means you have the throttle open (pressure is opposite of vacuum since vacuum is simply a measure of pressure....low pressure = high vacuum and high pressure = low vacuum).

Normally a fuel injected car used the MAP sensor to help gauge throttle position (among other things) in an attempt to guess how much fuel the engine needs.

On the Protege the MAP sensor isn't used for the fuel injection system but solely to test the operation of the EGR, since, if all things considered you have the throttle a position X then the pressure reading should be Y. If the engine computer tells the EGR to open then the pressure should change and this change is read by the MAP sensor. On the Protege the MAP sensor reads vacuum/pressure from a small port on the back of the intake manifold just behind where the EGR mounts. If that port is blocked then the MAP sensor cannot measure any changes and you get a thrown code. In short the code says bad MAP sensor but really what is happening is the MAP sensor isn't getting the readings it should. Most likely your MAP sensor is fine.

As well you could get the same code if your EGR isn't closing or opening fast enough, and, a clogged or malfunctioning EGR could cause the very symptoms you describe (RPMs failing to correct themselves since an engine at low RPMs is more intolerant of a vacuum leak...which is what a stuck EGR valve acts like until it closes or the engine adjusts by using more gasoline).

Sounds like to me you need to pull off your EGR and have a look inside the manifold. Chances are you might need to pull the manifold and clean out the entire EGR passage along with the EGR itself.

[cdgator]

i got an intake manifold clean professionally done and the car did run better. So when i got that cleaning wouldn't it have cleaned the egr as well since isn't it on the back bottom of the intake manifold. If not how hard is it to get the egr off i saw the thread on how to clean in and whatnot.

[smokstac]

To have it cleaned properly would require removal of your entire intake manifold and having the passages cleaned out.

If someone sprayed something into the intake while the engine was running it did nothing for the EGR or its passages.

[cdgator]

okay well i don't have the knowledge or correct tools to take the manifold off. Would a regular shop take it off and clean the egr? Or should i take it to a certain type of shop?