Miss firing?? Not sure!!!

SVXgarcia · #1 11-07-2009, 09:09 PM

Year:1992 SVX
Mileage: 135000

When car goes from zero mph I feel the car lacking power, even when I accelerate fast.
What exactly do I mean by lacking power, i will try to explain:
At take off the car feels almost as if the throttle cable was getting stuck and suddenly releases, causing the car to accelerate quickly. I can depress the pedal and the car will not go anywhere. Then suddenly the car accelerates.
When this happens I hear something clicking on the engine at the same time that the car quickly accelerates. Almost as if something was unclogging and finally the car accelerates again.
Problem happens only when the car comes to a full stop and I have to start from 0 mph. Doesn't happen on the highway.

Thank you for any help.

SoCal LS-L · #2 11-07-2009, 09:52 PM

Look above in the How-To documents on how to test your throttle position sensor.... that would be my first step in solving your issue.

immortal_suby · #3 11-08-2009, 05:51 AM

Also check for "check engine" codes. There is a how-to document to retrieve stored codes.

SVXgarcia · #4 11-08-2009, 08:37 PM

Thank you I will try both and report back.

SVXgarcia · #5 11-09-2009, 01:28 PM

Checked the engine codes and only got code 37 for No.2 Left Oxygen Sensor. Highly doubt the sensor will cause the problem mentioned above.

SVXgarcia · #6 11-09-2009, 01:49 PM

Should I replaced the TPS?

Frank Rizzo · #7 11-09-2009, 02:51 PM

A faulty TPS usually doesn't cause a misfire. Just eratic shifting in my experience. You can check the TPS with a volt meter but in my opinion you'd be wasting your time.

It's most likely a coil wire. They go out frequently. You can easily diagnose what cylindar it is by disconnecting the coil wire connectors one by one and starting the engine. If the missing improves it's probably the cylindar in the same possition on the oposite side. If the missing gets worse it's not the one you disconnected. If it stays the same that could be the faulty coil wire.

SoCal LS-L · #8 11-09-2009, 07:25 PM

Quote:

Originally Posted by Frank Rizzo

A faulty TPS usually doesn't cause a misfire. Just eratic shifting in my experience. You can check the TPS with a volt meter but in my opinion you'd be wasting your time.

It's most likely a coil wire. They go out frequently. You can easily diagnose what cylindar it is by disconnecting the coil wire connectors one by one and starting the engine. If the missing improves it's probably the cylindar in the same possition on the oposite side. If the missing gets worse it's not the one you disconnected. If it stays the same that could be the faulty coil wire.

The OP stated that it "might be" a misfire, read the symptoms stated. The TPS is also responsible for notifying the ECU to advance the timing when the throttle is pressed..... and since his symptom is a complete delay in acceleration when the throttle is pressed, not rough running, I wouldnt suggest to him discounting an easy test such as verifying a smooth linear voltage output of the TPS.

oab_au · #9 11-09-2009, 09:42 PM

I don't think it is the TPS, it only tells the ECU how far throttel is open.
The MAF sensor is used for the main things.

I go along with a missing plug, maybe a few missing plugs. Could be why the O2 sensor is reporting a code.

Harvey.

SoCal LS-L · #10 11-09-2009, 10:34 PM

Quote:

Originally Posted by oab_au

I don't think it is the TPS, it only tells the ECU how far throttel is open.
The MAF sensor is used for the main things.

I go along with a missing plug, maybe a few missing plugs. Could be why the O2 sensor is reporting a code.

Harvey.

Ok, now correct me if im wrong, but isnt one of the reasons that the ECU needs to know how far the throttle is pressed in for the purpose of timing advance??? Acceleration depends on ignition timing advance, without a proper TPS reading, the ECU wouldnt know when to advance the timing to provide more power, hence the problem with major delay when he presses the gas pedal.

Am I alone here? I mean its not a hard test to do to check TPS operation..........

From what ive been taught, for a distributorless ignition system, the ECU relies fully on MAF/MAP for mixture, and Crank/Cam + TPS sensor readings to calibrate advance, THEN adjusts fuel trim via O2 readings

SoCal LS-L · #11 11-10-2009, 08:46 AM

Just another thought on the matter of misfiring, if it IS a misfire......

If the TPS is telling the ECU it should be in idle mode, which would have the IAC valve open..... then he steps on the throttle..... if the TPS is faulty the ECU would not know to advance timing AND would make the engine run lean since the IAC would not shut until the TPS returned a voltage indicating the throttle is pressed. That would cause his hesitation AND a misfire, and could also explain an O2 code.

oab_au · #12 11-10-2009, 02:55 PM

Quote:

Originally Posted by SVXgarcia

Year:1992 SVX
Mileage: 135000

When car goes from zero mph I feel the car lacking power, even when I accelerate fast.
What exactly do I mean by lacking power, i will try to explain:
At take off the car feels almost as if the throttle cable was getting stuck and suddenly releases, causing the car to accelerate quickly. I can depress the pedal and the car will not go anywhere. Then suddenly the car accelerates.
When this happens I hear something clicking on the engine at the same time that the car quickly accelerates. Almost as if something was unclogging and finally the car accelerates again.
Problem happens only when the car comes to a full stop and I have to start from 0 mph. Doesn't happen on the highway.

Thank you for any help.

A couple of questions.

Does it do the same thing if you press the accelerator gently?
Does it do the same, if the engine is cold in the morning, or when it is warmed up?

Harvey.

oab_au · #13 11-10-2009, 03:16 PM

Quote:

Originally Posted by SoCal LS-L

Ok, now correct me if im wrong, but isnt one of the reasons that the ECU needs to know how far the throttle is pressed in for the purpose of timing advance??? Acceleration depends on ignition timing advance, without a proper TPS reading, the ECU wouldnt know when to advance the timing to provide more power, hence the problem with major delay when he presses the gas pedal.

Am I alone here? I mean its not a hard test to do to check TPS operation..........

From what ive been taught, for a distributorless ignition system, the ECU relies fully on MAF/MAP for mixture, and Crank/Cam + TPS sensor readings to calibrate advance, THEN adjusts fuel trim via O2 readings

Chris Ignition timing advance has more to do with the amount of air in the cylinder, and this is obtained from the Mass Air Flow. Throttle position is really secondary.

The engine will still accelerate, even if the spark does not advance, maybe not as well.

It's the amount of fuel/air in the cylinder that is ignited that accelerates it.

Quote:

If the TPS is telling the ECU it should be in idle mode, which would have the IAC valve open..... then he steps on the throttle..... if the TPS is faulty the ECU would not know to advance timing AND would make the engine run lean since the IAC would not shut until the TPS returned a voltage indicating the throttle is pressed. That would cause his hesitation AND a misfire, and could also explain an O2 code.

No it is the MAF that measures the amount of air that the ECU injects fuel for. The IAC does not introduce any other air, it all comes through the MAF, so it won't run lean, regardless if the IAC is open or closed.

Harvey.

SVXgarcia · #14 11-11-2009, 10:24 AM

Quote:

Originally Posted by oab_au

A couple of questions.

Does it do the same thing if you press the accelerator gently?
Does it do the same, if the engine is cold in the morning, or when it is warmed up?

Harvey.

Yes, it happens when press gently or fast

It does it both when warm and cold.

I will report back as soon as I get a day off to do some troubleshooting.
Thanks for all the valuable information.

SoCal LS-L · #15 11-11-2009, 11:20 AM

Quote:

Originally Posted by oab_au

Chris Ignition timing advance has more to do with the amount of air in the cylinder, and this is obtained from the Mass Air Flow. Throttle position is really secondary.

The engine will still accelerate, even if the spark does not advance, maybe not as well.

It's the amount of fuel/air in the cylinder that is ignited that accelerates it.

No it is the MAF that measures the amount of air that the ECU injects fuel for. The IAC does not introduce any other air, it all comes through the MAF, so it won't run lean, regardless if the IAC is open or closed.

Harvey.

I agree about the IAC, I should stop posting in the morning before my coffee.

However on the matter of ignition advance, I dont believe the TPS has less effect on timing than the MAF, as the ECU cannot determine engine load by airflow alone, or else climbing a hill at full throttle 2000 rpm would give the same spark advance as cruising half throttle at 4000rpm. If the ECU didnt adjust timing to compensate for a hard throttle, there would be a marked delay in acceleration until the MAF signaled the higher airflow, one of the symptoms listed. Timing is just as important as the size of the A/F charge, as if the intake charge is detonated too late it simply chases the piston down the cylinder instead of pushing it, making little to no power.

I have no doubt that you are knowledgeable in auto repair Harvey, and I may be wrong, it happens....... I just think he should maybe rule out the TPS since the test is extremely easy and the TPS is one of the more accessible components.