bouncing idle at start up

[stock_am]

I have a 1999 grand am with the 3400 engine. i recently did the fuel pressure regulator, spark plugs and wires. The problem i'm having is a bouncing idle at start up. Sometimes this will cause my car to stall. I also have no check engine lights or service lights that come on.

[rixGAphx]

Erratic idle, sometimes stalls, no SES light.

Probable vac leak (on the non-emissions system), possible dirty air filter or clogged cat.
* Check the vac tubing, devices, and connections for leaks; especially check the vac tubing at the vac reservoir, black plastic grapefruit-sized ball under the battery.
Are the HVAC controls doing anything funky?
The emissions tubing and devices are shown on the decal on the passenger front strut tower; these will always show an SES light if faulty, so don't bother with them.

* Inspect the air filter; clean or repalce as required.

* The cat can be partially-clogged and cause trouble at idle, yet work well at higher rpm and clean well at all times (thus the SES isn't 'on') .
A muffler shop can burn a 1/4" hole in the exhaust pipe a few inches ahead of the cat to check for clogging; then weld the hole shut if that's not the problem.
You can do the same thing with a drill and a large stainless steel self-tapping sheetmetal screw.

Hope this helps,
-Rick

[HeyDace]

Rick is correct, possible vac leak. One other possibility is a dirty/bad MAF or dirty throttlebore which may not trip the light.

[SE2000]

Did you do the stuff to try to fix the problem or after you did it, you had a problem?

[stock_am]

Originally posted by SE2000
Did you do the stuff to try to fix the problem or after you did it, you had a problem?

yes i changed the fuel pressure regulator because of the problem. I know spark plugs and wires have nothing to do with the problem. I forgot to say that i did change air filter and also did clean the throttle body. i've had this problem ever since i bought the car. It could be the MAF, but from what i know you can't clean that. i've tried checking for vacuum leaks b4 but couldn't find anything.

[rixGAphx]

Originally posted by stock_am
i've tried checking for vacuum leaks b4 but couldn't find anything. Get a spray can of Winter Starting Fluid.

With engine warm and idling, spary at each vac line, device, and connection.

A vac leak will suck-in the stuff, and the idle will rise and smooth-out.

Hope this helps,
-Rick

And don't forget to check for a clogged cat.
At 8 yrs old, about 100k miles, the cat has lasted its 'service life' and may well be clogged.
Especially if the engine had a rich-running condition for some time in its past.

I guess there could be some OTHER sort of obstruction in the exhaust, like somebody rolled a golfball in the tailpipe.

[stock_am]

ok i'll do that when it gets warmer a bit. i'll check for the clogged cat also.

[stock_am]

UPDATE: This problem is getting worse. Now my car won't start when its warmed up unless i give it gas. As soon as i start it the revs go up and down. Another thing that is happening is that the car feels like its gonna stall everytime i let go of the gas. I hooked up a tech 2 to it at school and it has 13 IAC counts at idle and -19 on the long term fuel trim. I also hooked up a fuel guage to the car and as soon as you turn off the car the fuel pressure drop almost righ away. The fuel pressure while running is 48PSI which is within spec. What could this be????

[rixGAphx]

Originally posted by stock_am
UPDATE: This problem is getting worse. Now my car won't start when its warmed up unless i give it gas. As soon as i start it the revs go up and down. Another thing that is happening is that the car feels like its gonna stall everytime i let go of the gas. I hooked up a tech 2 to it at school and it has 13 IAC counts at idle and -19 on the long term fuel trim. I also hooked up a fuel guage to the car and as soon as you turn off the car the fuel pressure drop almost righ away. The fuel pressure while running is 48PSI which is within spec. What could this be????
When the engine is cold, the PCM is trimming the fuel according to 'open-loop' preset values.

When it warms up, the PCM senses the O2 sensor and trims the fuel injection according to the richness/leaness of the exhaust. Thus the bouncing revs, as the PCM tries to hit a moving target.

I'll bet one or more of the fuel injectors is partially stuck open, allowing a steady dribble of fuel into the respective intake runner/cylinder.

Ultra-rich condition, solved by pressing on teh gas pedal and opening the throttle so the air-fuel mixture becomes more reasonable.

For some reason, #2 seems to be the injector most-prone to sticking-open on the GA V6.

Try a couple bottle of FI cleaner, they *may* dislodge the microscopic grain of sand that is jamming the injector needle open.

-Rick

[pokesmot]

Originally posted by HeyDace
Rick is correct, possible vac leak. One other possibility is a dirty/bad MAF or dirty throttlebore which may not trip the light.


:agree:
im leaning towards MAF too.

couple things to try, first obviously inspect the connections to the MAF (they rarely fail but might as well start with the obvious)

tap on the MAF sensor with the car running with a screw driver lightly a couple times. If the car faulters / stalls your MAF is bad.

The MAF isnt "supposed" to be serviceable.. ford attempts to discourage service of their MAF's by using solder on the torx screws that hold it in... but you can disassemble them and clean them.

At work we usually just use carb cleaner and blow dry it VERY lightly with compressed air

hope this helps.

edit: just remember the MAF is very delicate and the utmost care must be taken while servicing.

[stock_am]

Originally posted by rixGAphx
When the engine is cold, the PCM is trimming the fuel according to 'open-loop' preset values.

When it warms up, the PCM senses the O2 sensor and trims the fuel injection according to the richness/leaness of the exhaust. Thus the bouncing revs, as the PCM tries to hit a moving target.

I'll bet one or more of the fuel injectors is partially stuck open, allowing a steady dribble of fuel into the respective intake runner/cylinder.

Ultra-rich condition, solved by pressing on teh gas pedal and opening the throttle so the air-fuel mixture becomes more reasonable.

For some reason, #2 seems to be the injector most-prone to sticking-open on the GA V6.

Try a couple bottle of FI cleaner, they *may* dislodge the microscopic grain of sand that is jamming the injector needle open.

-Rick

yesterday when my brother took the car to work its started to idle about 3000rpm and wouldn't go down, would an injector also cause this problem??? I'm also thinking if this is a PCM issue.BTW i also changed the TPS and IAC on the car.

[stock_am]

2ND UPDATE: The problem is solved, all this stupid stuff that was happening was all because of the fuel pump. My car now starts awesome no more bouncing idle and has alot more power. Down side was that it cost $1000 to fix.

[Norco]

hope my fuel pump not going lol, doesnt sound quite as bad as what u got but its a start, i have erractic idle when i start car sometimes which goes away after 30-60 seconds or if i shift outta park goes away instant, car has had this for 3 years

[rixGAphx]

Originally posted by stock_am
2ND UPDATE: ...the fuel pump.
Down side was that it cost $1000 to fix. 1000CDN ?
:ouch:

Brand-new ACDelco FP thru NAPA = $250 US; call it 400US over the counter at a GM dealer . . . . . . . . =500CND
Analysis/troubleshooting = 2 hours. . =200CND
R&R fuel tank and FP = 3 hrs. . . . . . . =300CND

Yup, I guess that's what it is.

Glad you're rolling powerfully again,
-Rick