Very rough idle

[Jscott]

Hi again. 90 old C/S 3.1. Really bad idle. I replaced the o2 sensor and iacv. After installing the o2 it ran pretty good and then it didn’t. Back to rough idle. Any thoughts on what the issue could be? I’ve heard of bad pcv/ map sensor/ coil pack. The erg is not gummed up. However I just had my battery tested and came back bad. The car starts fine but they said it was bad. I’m not sure if that has something to do with it. Any ideas. Thanks. 

[55trucker]

My suggestion to you is to not throw parts at the issue, get a proper scanner on the engine, do a live readout & note in particular what the a/f display is showing you.

A failed o2 sensor will only prevent the fuel system from going into closed loop. If the sensor HAD failed then something upstream caused it to.

A check engine light will also light up. A bad map will completely throw off the ECM;s ability to configure a proper air/fuel ratio, that will be seen on a scanner as well.

Another small item to look at is the wiring harness condition to the engine temp sensor, if it's faulty then that sensor cannot send the proper signal to the ECM to configure A/F ratio due to inaccurate information on the engine actual temp. That will be seen on a scanner as well.

The 3 vital sensors needed for the ECM to calculate proper A/F ratio when the engine starts are the MAP, IAT, ENG TEMP.

 

  

 

 

[Jscott]

Great advice. Thank you. 

[55trucker]

You're welcome....try to resist the usual approach of throwing money at a problem.

 

[Jscott]

I get it, I don’t want to shoot the parts cannon at it. lol. And suggestions on a live read scanner. Do they sell something like that for OBS1’s

[55trucker]

You're more than likely going to find yourself on ebay looking for one, everything in the market today is geared to OBD2.

Some older models would read OBD1 & OBD2 up to a given year depending on the plug in cartridge.

The interface for OBD1 was not a single standard plug, it varied on the vehicle manufacturer & so did the harness to the scanner.

The item I own is an OTC 4000 enhanced.

 

https://www.ebay.com/sch/i.html?_nkw=otc+monitor+4000+enhanced&_sacat=0&_from=R40&_trksid=p2334524.m570.l1311&_odkw=otc+monitor+4000&_sop=12

[Jscott]

I have the little Gm plug in style. And the code comes up as a 35 and in the code book says ( IAC system problem-can not set desired RPM. In service manual it says that it has to be relearned with the Tech 1. I was wondering if there was a way around that. 

[55trucker]

Ok, that sounds like what you have is a *code reader*, it's not a scanner.

A proper scanner is more advantageous to one. The scanner will clear that code, the scanner will run a diagnostic on all of the sensors in the system. It has *realtime* capabilities, as for the IAC relearn this scanner will go thru the steps needed.

Look on ebay for a used complete scanner kit, the replacement for the OTC 4000E was the OTC Genisys SPX system, a newer version, capable with both OBD1 & pretty much all OBD2 up to 2010 until the EVO replaced it.

Even if it sounds like a somewhat expensive initial outlay a scanner is a worthwhile tool to own.