Hello everyone, I have a 2001 blazer ZR2,I have been chasing this problem for two years now. I have been methodically as I possibly can be in testing just about every system in the vehicle. I did all the typical stuff with the proper parts such as ac delco ignition parts and denso o2 sensors. It’s just waaay too much to list here, you name it I have replaced it. I have no trouble codes and it starts right up. There is definitely a shake of the engine at idle in park when the engine is warm. When I give it gas it goes away and the rpm’s don’t fluctuate and are about at 600rpms if anything I think it’s kind of low. The only thing I could think of left is maybe it’s compression when hot as I did have great compression when cold when I tested it. I did however not check it when warm. Also, maybe carbon buildup in the intake as the throttle body butterfly was caked with carbon and I cleaned it. The spider was replaced by the previous owner with a high quality standard motor products brand spider injection mfi system. Maybe the plenum gasket wasn’t replaced

I am scared of using Seafoam as I don't want to hydrolock my engine. If I were to have to say in terms of fuel trims it’s definitely running lean and the plugs look a little whitish grey. It feels ok on lower speeds from a start especially when cold as I know there is a larger demand for fuel when cold and in open loop, which is masking the problem. It can’t be a vacuum leak as I don’t hear any hissing, I did also spray carb cleaner around the plenum. In addition to when warm as I mentioned in the title it can be at times intermittent such as if I go into a store and come back out and restart the engine will suddenly start shaking

But the WOT especially on the highway for passing is kinda of is weak for a 4.3 as I have owned 4.3 motors before and it should have more pep. Sorry for the long post, I would greatly appreciate some help

 

I would have a scanner with live data at the ready and see what cylinder misfires look like between running good and running crappy.

What do your fuel trims look like?

What is your CMP retard value?

If you replaced the crank sensor, did you do a relearn?

George

 

Thank you for your response, here is what I found

1) The misfire count seems to hover around 5-10 at idle and bounces around and goes back to zero with really no specific pattern . On the highway at 55 as long as I don’t let off the pedal I can keep the count to zero. I forgot to mention that this all started with the infamous P301 code that I gradually was able to stop from triggering from gradually replacing parts. It really went away for good after I replaced my catalytic converter which was plugged. I hear from people saying that these engines can normally misfire, however it trips a code only when it gets to a certain percentage.

2) The fuel trims are within the (plus or minus) 10%.
with the exception of bank1 LTFT which will occasionally at random increase to like +15 for a very brief second and quickly go back to normal values. Again this really has no pattern and like I said can happen at random.

3) I did change the crank sensor and perform a relearn procedure in which you rev the engine. However, it worked for a half a day and left me stranded as I had difficulty in shimming the sensor correctly from confusing info on how to perform this correctly . I just put my old sensor back in and left it alone.

4) The CKP retard value is between -1 and -2 and with no movement

 

1. Those are low misfire counts and probably insignificant on a high mileage engine, especially given crank sensor swapping and unsuccessful relearn attempts. Are they concentrated on any cylinders?
2. You must be watching fuel trims while driving or changing the rpm out of gear because at idle the LTFT numbers should not be momentarily changing. I am interested in idle and 2500 rpm out of gear values. You always add STFT and LTFT together for a given bank. Are any of those combinations above 15, absolute value?

On an older engine with minor rough idle issues you may be chasing ghosts but if it’s still important to try and work through this then :

A What are the results of a fuel pressure leak down test. What’s the history on the spider.

B You need to do a compression test. What do the spark plugs look like?

C Try misting the engine with water in the dark when it’s running rough to see if you see any arching

D Were any ignition parts replaced with non-AC Delco branded parts?

E Did you confirm that your exhaust back pressure values are within spec after the cat replacement?

F If the rough behavior is related to throttle position changes, monitor the TPS value through various throttle changes and look for a glitch in reported value

G If none of these things solves your problem then you will need to do a dynamic compression test to look for more subtle engine issues most likely related to engine wear

H Does a vacuum gauge show any wonky behavior?


George

 

1. I did perform a relearn procedure successfully in between swapping crank sensors. However it caused an intermittent no start because it wasn’t shimmed right or it could have been a defective sensor out of the box. It seems that the misfire counts are focused on cylinder 1
2. I checked the fuel trims in park at idle and they just occasionally just hop around with really no consistent pattern. I did manage to rev the engine to 2500 in park and get the LTFT on bank 1 to max to 16 for several seconds. The STFT seems rather much lower like within plus or minus 5% and stays consistent.

A) I am getting 55 psi fuel pressure with KOEO and about 50psi at idle. I then shut the engine off and then notice that the 50psi tends to drop about 5psi every 10 mins. I did notice that when I just cycled the key to reprime the pump in which it builds back to 55 psi and then watch the gauge as it then doesn’t drop any pressure at all over the course of 30 mins

B) I did a compression test when it was warm and all the cylinders were like 2psi less than the previous cold test that I performed in which all the cylinders were 180-183 psi. Attached is a picture of one of the spark plugs from cylinder 1 and the other 5 remaining

C) I tried misting around the wires and could not get anything to change and did not see any arcing. If anything it runs better when cold and damp

D) All the ignition parts are AC delco except the coil which is a Delphi and the module is a United Motor Products.

E) For exhaust back pressure I hooked up a vacuum gauge to an intake vacuum source and when warm I’m getting 20 psi at idle and when I snap the throttle it decreases to 10 and jumps back to 20psi

F) I checked the throttle position sensor for power and ground and signal. The signal for sensor back to the ecu is increasing smoothly in voltage when the throttle opened from closed to wot.

G) As for a dynamic compression test, does this mean compression is done while engine is running??

H) The vacuum gauge is pretty steady and not bouncing around



Lastly I would like to mention that I am getting a reoccurring p420 catalyst efficiency code. I did notice that I was able to erase it and drive around for a month but it then it came back. I know I should have used a more factory like converter, but this was all I could afford at the time as it was a universal one I did notice that I did not have this occur before I changed the converter and I needed my car in a hurry. I have been able to get it inspected at times when the catalyst monitor is not set and the rest are good. Honestly it’s a jungle out there trying to find a catalytic convertor that is ny compliant and not counterfeit . It also doesn’t seem to affect the power of the engine. The o2 sensors are also new denso sensors.

 

1. Was the relearn successful when you went back to the OEM sensor?
2. For each bank, what is the combined value of STFT and LT FT, mindful of sign? LTFT hopping around at idle is not normal.

A The proper way to test fuel pressure is just key on and then wait 10 min and measure the leak down value. Is this the old or new style spider? Depending on that answer, you may need to go to the fuel filter and retest.

B That’s a remarkable compression test result for an older engine. That said something dramatic is going on with cylinder one because that plug is so clean. Either it’s not fire correctly or there’s a problem with fuel delivery. Swap that plug with one of the other ones and see if it stays clean in the new hole. I would also look at the routing for spark plug cable number one. then check the spark on cylinder one and see if it’s strong and regular.. Do you have any carbon traces under that distr cap? If I had to guess I would say it’s fuel delivery and the injection on that cylinder.

Given the history on that catalytic converter and that code, you need to do a proper back pressure test in one of the oxygen sensor ports. Also, you should graph the O2 sensors in live data. You’re probably gonna see some out of limit movement on that rear sensor. Something killed the old catalytic converter and it’s usually a Rich mixture, coolant or oil in the exhaust and so you may still have that original problem. You should probably do a coolant system, pressure test and watch that gauge while the engine is running although once again, this is probably not the issue given those compression test results . I would imagine you don’t burn much oil with those compression test readings. I’ll go find the link for Les’s favorite dynamic compression test, but I think you have plenty to work on before you go to all that trouble.

George

 

My internet is out and I can’t copy and paste that URL.Look up All Data Running Compression Test

George

 

Got my internet back up and running:

https://www.alldata.com/us/en/suppor...pression-tests


George

 

Hey George! I've been contemplating what might be a better way to check for restricted exhaust. Let me run it by you and see what you think.

Suppose we use the MAF sensor reading to calculate volumetric efficiency at say full throttle 4000 rpm in second gear (held back by brakes). If exhaust is restricted, volumetric efficiency will be down.

Only questions are if the MAF reading is accurate enough and what is normal.

Would be super easy to check with most any scanner.

What do you think? Are we interested in pursuing this?

 

Low VE would indicate a problem and high back pressure would be one candidate among others. So it would not be a definitive test for a back pressure problem but passing with flying colors would be enough to rule it out. What is your thinking regarding the brake thing, why not just a full throttle shift point reading while driving?

It's a good test in any event.

George