Car still shuttering/accelerating poorly/perhaps misfiring? Need more ideas!

[brtnstrns]

I've posted about this before and have been given some ideas but so far my car is still acting strange.

I'll describe whats happening:
The engine INTERMITTENTLY becomes incredibly bogged down. I'll press on the accelerator and the engine makes almost no power and begins to shutter as if the clutch was messed up (I've been told its not likely the clutch due to the intermittent nature of it). During this time the exhaust also begins to sound like a diesel (I have flowmasters but they definitely do not make that sound normally). At about 2000 RPMs the engine seems to go back to normal...until the next gear. This problem happens in every gear and happens very randomly. It will sometimes happen on my drive somewhere, but not on the way back. The engine will sometimes act fine, then transfer into this problematic performance. There has been no noticeable specific variables which make the engine behave this way: it can happen on a cool morning, hot afternoon, highway driving, city driving, short drives, long drives...doesn't matter. It seems very random.

I had idling problems in the past which brought up a code about the drivers side hot O2 sensor. I replaced that O2 sensor and the idling problems went away. Now this is happening. During the time that the poor idling was going on, I checked/replaced many things that I was told could be the culprit. This is the list so you know what I've done so far and what should be eliminated from the possible culprits for this new problem:

-Replaced driver side hot O2 sensor
-Check all vacuum lines (that I know of) for any leaks, etc.
-Checked EGR valve functionality/tightened fittings connected to EGR
-Replaced all spark plugs
-Cleaned MAF sensor
-Replaced fuel filter

I'm going to replace the passenger side hot O2 sensor today to see if anything changes since the car has 117k miles and it wouldn't hurt to replace that one since I've replaced the drivers side a few months ago. The car is not throwing any codes on the dashboard and I don't have an ODBII scanner to check of any "pending" codes. If anybody has any ideas on what else I could check out, I'm all ears.

Thank you all

 

[trinity_gt]

Check the fuel pressure at the rail, preferably as you drive and experience the bogging.

 

[bigkeeko]

Have you had a scanner on it since? Any codes?

Fuel line blockage? Injector problem? Has it been re-mapped/re tuned or had parameters changed?

Have you ran an injector cleaner? or had any of the fuel supply parts changed? Fuel pump ok?

Just trying to get an idea here.

 

[brtnstrns]

Have you had a scanner on it since? Any codes?

Fuel line blockage? Injector problem? Has it been re-mapped/re tuned or had parameters changed?

Have you ran an injector cleaner? or had any of the fuel supply parts changed? Fuel pump ok?

Just trying to get an idea here.

I have not gotten a chance to put a scanner on it since the initial idling problem.

Car has not been tuned. What is the best way for checking fuel line blockage/faults at the injectors?

I don't remember putting fuel injector cleaner in it - if so it would have been a long while ago. No fuel supply parts changed. Not sure about fuel pump - how would I check that?

Thanks!

 

[brtnstrns]

bump

Well, as I predicted, replacing the O2 sensor did not fix my problem.

 

[brtnstrns]

Could anyone give me advice on how to check my fuel delivery system in order to see if that is the cause of this problem?

 

[trinity_gt]

Check the fuel pressure at the rail, preferably as you drive and experience the bogging.

 

[joshjwc9]

Check the boots on your COPS. There could be one that is arcing to the motor causing a misfire, and this usually does not throw codes. Ask me how I know...and make sure all are seated completely on the spark plugs. It could be a injector not pulsing correctly, none of the plugs smelled extremely like gas? Are the wires for all of the o2's intact? Some may accidently get caught on the exhaust and burn.

 

[brtnstrns]

Check the fuel pressure at the rail, preferably as you drive and experience the bogging.

What do I use to check the fuel pressure?

 

[wmburns]

The motor sounding like a diesel could be the PCM taking away timing advance. The trick is to find out why. To which having an ODB2 scanner than can monitor operational data would be a HUGE help.

A couple quick causes come to mind.

Does your car have traction control? Would be handy to monitor timing to see if the PCM is taking away timing. This could indicate a false signal from a wheel sensor.

Fuel delivery problem. Use an ODB2 scanner to monitor fuel pressure as reported by the PCM. What were are looking for is a drop in fuel pressure when the problem occurs. If the fuel pressure is dropping, the next trick is to narrow down WHY.

Bad or loose connection to the MAF. Use an ODB2 scanner to monitor air flow. What we are looking for is the MAF signal to suddenly drop when the problem happens.

Bad or loose connection to the CKP sensor. Use an ODB2 scanner to monitor RPM's as reported by the PCM. If the CKP signal is poor, this will show up a rapidly changing RPM's.

Bad/stuck EVAP VMV valve. This can be ruled in/out by disconnecting an plugging the EVAP line to/from the intake. It will throw an EVAP related DTC, but it's OK for testing.

EGR problem. Even though you have checked the EGR value, you have not confirmed the EGR flow when the problem is happening. Have you inspected the vacuum lines to/from the DPFE sensor? How about the electrical connection to the DPFE sensor?

Perhaps do a low tech check by disconnecting the EGR value vacuum line. This will disable the EGR system. This will cause a EGR related DTC, but it's OK for testing. What we are looking for is a change in symptom.

If this were an ignition misfire, IMO there would be pending codes indicating this. For this reason, IMO this is less likely.

It would also be handy to monitor BATTERY VOLTAGE when this problem is happening. Today's car MUST have a stong charging system with stable voltage.

Again, an ODB2 scanner/data logger would likely make short work of this. The cost of parts already spent would likey have paid for the scanner.

 

[brtnstrns]

I'm borrowing an OBDII scanner that monitors real time data. However, it doesn't read off the absolute fuel rail pressure for some reason (in the manual it says its one of the things the scanner reads - could be related to the scanner not recognizing the vehicle). It does show long term and short term fuel trim...is that of any relevance?

 

[brtnstrns]

Edit: spoke too soon