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Old 05-04-2012, 10:13 PM   #1
comanche
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2001 SL
Default '00 DOHC - rough idle, low power, no codes

My brother has an '00 SL2 with 180k on it
Recent valve job & gaskets due to low compression issues.
Replaced timing chain, tensioner, guides at same time.

Now has rough idle and low power, but no codes present.
Data, as seen on OTC Nemisys updated to 2008.
Fuel trims are +25-50 short term, +25 long term
O2 sensor 1 functioning normally
At idle, manifold pressure 12-13 inHg (our other Saturns are between 8 and 9)
Injector PWM 3.14 on average
ECTS is newer and brass
Air fuel ratio is showing 14.7:1 but the system may be compensating to create this value
EGR 0% actuated
IAC 43 counts or something like that (normal if compared to our SC2 that runs great)
Idle does not improve with O2 sensor 1 removed from manifold
Idle does not improve much (increases a little) when brake booster hose is removed

I am leaning towards the possibility that the timing chain was installed with perhaps one tooth off.
Should I be considering something else?

Thanks in advance!

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Old 05-05-2012, 07:56 AM   #2
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2003 L-Series 3.0L Sedan
Default Re: '00 DOHC - rough idle, low power, no codes

1-Perform another compression test. This can draw a better picture on whether or not a valve job corrected original issues. Not doing one will leave you guessing.

2-With a new coolant sensor, was the thermostat ever replaced with another one?

3-At start up, what's idle rpm? When warmed up?

4-Where does the temperature needle move to?

5-Was the throttle body cleaned?

Quote:
At idle, manifold pressure 12-13 inHg (our other Saturns are between 8 and 9)
Injector PWM 3.14 on average

Idle does not improve with O2 sensor 1 removed from manifold
Idle does not improve much (increases a little) when brake booster hose is removed
6-Where did you attach the vacuum gauge to record (lower than normal) readings? Idle vacuum should be between 15-18in HG. Your readings suggest a vacuum leak somewhere but without the higher rpm associated with intake air leaks.

Pulling off the vacuum brake boost hose usually causes a runaway engine to rev immediately. Every one that does major engine repairs and overlooks this large vacuum hose (falling off or left off) on the back of the engine intake manifold gets the runaway engine at first start up, guaranteed.

Pulling off the front O2 sensor is used to determine a plugged catcon. An immediate drive around with the LOUD exhaust either shows dramatic engine performance from an alternate exhaust path or nothing.

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Old 05-05-2012, 10:57 PM   #3
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Default Re: '00 DOHC - rough idle, low power, no codes

Quote:
Originally Posted by fdryer View Post
1-Perform another compression test. This can draw a better picture on whether or not a valve job corrected original issues. Not doing one will leave you guessing.

2-With a new coolant sensor, was the thermostat ever replaced with another one?

3-At start up, what's idle rpm? When warmed up?

4-Where does the temperature needle move to?

5-Was the throttle body cleaned?



6-Where did you attach the vacuum gauge to record (lower than normal) readings? Idle vacuum should be between 15-18in HG. Your readings suggest a vacuum leak somewhere but without the higher rpm associated with intake air leaks.

Pulling off the vacuum brake boost hose usually causes a runaway engine to rev immediately. Every one that does major engine repairs and overlooks this large vacuum hose (falling off or left off) on the back of the engine intake manifold gets the runaway engine at first start up, guaranteed.

Pulling off the front O2 sensor is used to determine a plugged catcon. An immediate drive around with the LOUD exhaust either shows dramatic engine performance from an alternate exhaust path or nothing.
1. Will do.
2. I believe so, I will double check. Scan tool shows normal readings on startup and warmed up. 205 was highest reading seen.
3. Will verify. I think I remember seeing desired idle on the scan tool being 965, but that may have been when it was cold.
4. See #2. I'll have to check.
5. No
6. Vacuum readings were from the scan tool. Where is the proper vacuum port on these cars? I don't think there is one.

Pulling off the brake booster hose does cause an immediate jump in idle, but not as much as some cars.
We may have to drive the car with the O2 sensor removed. I do know that the sound when it was removed was MUCH softer than when I had a verified plugged cat and removed the sensor, so I don't think we have a plugged cat here. Besides, it ran great before the repairs.

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Old 05-05-2012, 11:13 PM   #4
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Default Re: '00 DOHC - rough idle, low power, no codes

There are no 'proper vacuum port' for engines. Use the pcv valve port or the vacuum brake boost port on the back of the intake manifold with an analog pressure gauge.

While its nice to use an electronic reader to interpret manifold vacuum, this implies using the map sensor as the electronic equivalent of a vacuum gauge. What if the map sensor were faulty? Depending on it can falter and you may not know it. It may not trigger a map sensor error code. And..........it may be the faulty part contributing to this issue.

A perfect example of electronic dependence on a faulty part is the infamous coolant sensor that outputs incorrect signals but the pcm doesn't know any better.

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Old 05-09-2012, 08:17 AM   #5
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Default Re: '00 DOHC - rough idle, low power, no codes

Quote:
Originally Posted by fdryer View Post
There are no 'proper vacuum port' for engines. Use the pcv valve port or the vacuum brake boost port on the back of the intake manifold with an analog pressure gauge.

While its nice to use an electronic reader to interpret manifold vacuum, this implies using the map sensor as the electronic equivalent of a vacuum gauge. What if the map sensor were faulty? Depending on it can falter and you may not know it. It may not trigger a map sensor error code. And..........it may be the faulty part contributing to this issue.

A perfect example of electronic dependence on a faulty part is the infamous coolant sensor that outputs incorrect signals but the pcm doesn't know any better.
Compression numbers are all 135-150......... ouch.
Yet they were at 180 after the valve job, with the manifolds removed.
Idle is around 1000 when started and a bit lower once warmed up.

I'm leaning towards something mechanical, since the car ran flawlessly until the day of low compression.

Haven't been able to check "real" manifold pressure yet.

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Old 05-09-2012, 08:43 AM   #6
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Default Re: '00 DOHC - rough idle, low power, no codes

The "Recent valve job & gaskets due to low compression issues" doesn't address worn out piston rings hence the suggestion to gauge compression again (in post #2). With low compression numbers, this suggests worn rings. To remove all doubt, put some oil into each cylinder to perform a wet compression test. If numbers increase dramatically then the rings are worn out.

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Old 05-10-2012, 02:57 PM   #7
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Default Re: '00 DOHC - rough idle, low power, no codes

Quote:
Originally Posted by fdryer View Post
The "Recent valve job & gaskets due to low compression issues" doesn't address worn out piston rings hence the suggestion to gauge compression again (in post #2). With low compression numbers, this suggests worn rings. To remove all doubt, put some oil into each cylinder to perform a wet compression test. If numbers increase dramatically then the rings are worn out.
2 tbsp oil increases compression by about 30 psi/cylinder.

Vacuum reading is 15, which is symptomatic of late ignition timing.
Which is triggered by the crankshaft only on these models.

I wonder......... hmmmmppph
We have a few Saturn engines & parts laying around...
I wonder if someone grabbed the wrong crank pulley........................
That would be a dumb mistake but we'll hope that's the case. I'll have him check it out.

*** Actually, never mind. I was thinking of the Alero crank sensor. The SLs have the sensor in the back. Bummer. That sounded easy.

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Old 05-10-2012, 03:38 PM   #8
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Default Re: '00 DOHC - rough idle, low power, no codes

Increasing compression 30psi by adding oil simply means the valve job was only half the repair - the piston rings are worn out and the main cause of low compression. The cure is replacing the rings if the bores are clean and pistons are sound. There's also the modification of drilling a few more oil drain holes in each piston to allow better oil drainage.

Vacuum has nothing in common with ignition timing. Vacuum is all mechanical; the ability of all four pistons to suck air at closed or nearly closed throttle. The more closed the throttle plate is the more vacuum created at low idle rpm. Its all mechanics. Ignition timing does not alter vacuum - all timing is electronically controlled to make spark at the right 'time' for each piston; maximum advance timing to make the most power when all parameters are met, retarding timing when excessive engine load occurs or when detonation/knock is detected that can damage an engine. Spark timing relies on many sensor inputs to arrive at the ideal ignition point to make the most power for each combustion cycle. Timing doesn't affect vacuum and vice versa.

All base timing begins with the crank sensor detecting the machined 'teeth' on the crankshaft - this is all fixed and cannot be changed. When the crank sensor detects 'teeth' at minimum rpm, output signals are the timing signals to be used by the pcm to; turn on the fuel pump, initiate the ignition system for spark timing, and pulse the injectors. The cps originates all timing signals that allows the pcm to operate. A dead cps = a dead pcm.

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