domano 68

After cleaning the maf, tb, new plugs, and new pcv my sisters 99 still idles rough at lights. She has a cel w a p0138 code. I am not certain but is this the rear most o2 sensor? Also would a bad o2 sensor cause rough idle at lights( does not stall out)?

I have done an extensive search therefore the remaining questions. TIA!

jholley

iTrader: (13)

If you read the 'Check Engine Light' sticky you could of confirmed that P0138 (0510) does pertain to the downstream O2 sensor after the CAT. This is the info it contains regarding that cel code:

0510
Diagnostic Trouble Code 0510 points to a problem with the Rear Heated Oxygen Sensor. This sensor is mounted in the exhaust stream, just downstream of the Catalytic Converter. When the two Front Heated Oxygen Sensors are operating properly the Rear Heated Oxygen Sensor is not used for engine control operation. It is used to monitor the condition of the Catalytic Converter. If either or both Front Sensors fail the Engine Control Module uses the signal from the Rear Sensor to maintain a correct fuel/air mixture.

DTC 0510 is detected when the signal from the Rear Heated Oxygen Sensor does not reach the expected maximum voltage threshold value. The possible causes include ...
- Harness or connectors (the sensor circuit is open or shorted.)
- Rear Heated Oxygen Sensor
- Fuel pressure
- Fuel injectors
- Intake air leaks


Since you're getting a rough idle I would check the injectors, FPR, and for a vacuum leak.

A bad downstream O2 sensor has no effect on idle, but a faulty upstream O2 sensor would effect it.

cashoit

Since u a 99, i would say look at the ignition coils as those are notorious to fail on a 99. U can also test MAF too and IACV per FSM

domano 68

Thanks for your reply. I did check the post you mentioned but was not certain about rear u2 sensor being the downstream sensor mentioned in the post. What is FPR? And what to check with fuel injectors? Thanks again.

Caticorn

Downstream O2 sensor doesn't impede performance as it isn't used for engine control. If it's irregularly bumpy at idle it's probably coil packs.

domano 68

How much does a set of coil packs go for and where in the fsm does it describe how to test ( what section)?

The car has 70k miles (didnt mention before)
Thanks!

Shift_A32B

iTrader: (14)

Make sure all o2 sensors are working, they will create
rough idle; I am talking from experience not research. Clean out the egr and make sure the gasket is okay.

jholley

iTrader: (13)

Last weekend I reinserted an OEM downstream O2 sensor because the aftermarket O2 sensor I had in there was too big to fit with an O2 spacer I was installing. On a 52 mile trip the OEM downstream O2 sensor gave me a CEL CODE within a minute. One thing this faulty downstream O2 sensor didn't give is rough idle.

Sounds like you don't know that the downstream O2 sensor just measures the CAT. It can only effect the air/ fuel mixture if one of the upstream O2 is defective along with his downstream O2 sensor that is already defective.

joseph.mcgowen

my 96 is doin the same thing except im throwing codes its not the maf tb iacv or coils so i decide to check the fpr and the 2 screws holding it i were striped so i was just gona buy a fuel rail anyone kno how much one is and whee to get one they dont sell em at autozone

JNCoRacer

iTrader: (6)

Why not fix whats throwing the codes?

joseph.mcgowen

im not throwing codes my bad i didnt reread i was in a hurry when i posted it

mmgio

Does the tach go up and down when it's idling rough? What rpm is it at? Auto or manual?Did you clean the iac valve? Any missing while driving? Sorry for all the questions but these answers will help diagnose the problem.

joseph.mcgowen

im sorry for takin so long to reply been extremey busy
and my tach is always goin up and down it hasnt worked since i got the car, its an auto, and i cleaned the iacv and yes it does seem to miss while driving but no codes

redrock5432

Just a thought.. It is fairly straight forward to check coils. With engine running at idle, one disconnects each coil connector one at a time reconnecting after test and noting engine behavior changes. Each coil should contribute power, so engine should get rough as any one is disconnected. If no change is detected, it is likely that that coil & plug combination is not contributing power.

italianbonez718

i had the same problem on my 95 for a looooong time. switched the ignition coil on two cylinders and the problem was fixed. check your coils. it sounds like its acting like mine did after i got new spark plugs.

domano 68

What section and page in the factory service manual discusses how to test the ignition coils? Thanks.

redrock5432

I am looking at a 2004 service manual. The test was in the "Engine Control" section on page EC-254. Here is text:
This procedure works best for a hard/constant failure. For intermittent failures, it is better to have a known good coil to plug into cylinder indicated by code, p030x (x being cylinder).