Since the COVID lockdowns, I have not driven my Blazer around too much. Occasional trips to the office, running errands, visiting friends/family, etc...
Over the course of this past year, I've slowly began noticing a reduction in engine power. She just doesn't accelerate like she used to. I just thought the engine was just getting old, until I noticed a big reduction in MPG and fuel trims.
Before the COVID lockdowns, I was communiting to the office every single day and the MPGs would average around 200 - 250 miles before the 19 GAL fuel tank (2D LS Blazer) would be at 50%. Roughly 20 - 25 MPG mostly highway.
I recently took a road trip from Orlando FL to Savannah GA (most weekly miles it's gained in 2 years!), and my Blazer was hitting 50% fuel capacity at around 150 miles since last refuling. Something is clearly off.

I started investigating the low power problem, cause it's been on the back of my mind the longest. I read about back-pressure tests I can do, but I started knocking around the exhaust pipe and I heard a rattling inside. Clogged/damaging cat mesh I suspect. So I know something is up with the catalytic converter; no need to test back-pressure. I suppose I'll look into replacing that and perhaps the factory o2 sensors while I'm at it. Anything else I should look out for?

So now I'm wondering, why did the cat fail? For a short while I switched to ethanol-free gasoline (just to see any MPG difference), and I deffinately noticed strong exhaust odors before getting warmed up. Too much unspent fuel?

I've posted about fuel trim woes here before, and I have the data to support the claim that the fuel system is running rich. Not enough to trip a code, but enough to notice something. Short term seems fine, but long term fuel trims are showing -13% consistently. Dripping fuel injector? Would certainly explain the negative fuel trims, strong exhaust odors, and why the cat failed.

I haven't pulled the spark plugs yet to see their condition. I just installed new ones not long ago and they have less than 15k miles on them. I'll check those eventually and report back for remedies if needed.
If I do find spark plugs that are carbon fouled from the rich codition, I guess the best fix is to pull off the plenum and replace fuel injectors, right? Or should I just replace the whole spider assembly?

What do you guys think? Am I on the right track here? Fuel pressure is fine; holding 50 - 60 PSI consistently.

 

My underdtanding is that cats can go bad from vibration and fail mechanically irrespective of fuel.

A bad, clogged cat can affect mileage. But also the ethanol content can too. However, I'd fix a bad cat before I'd consider fuel delivery being bsd.

As for the O2 sensors, they can become fouled with carbon. They should be replaced along with the cat.

I could imagine a broken cat causing poor performance which then csuses the ECM to adjust the fuel trims to compensate which then could cause a fouling of the O2 sensors if the fuel is enrichened.

 

Replacing the cat is really expensive, so I'm going to hold out on that until I absolutely know it's causing a problem.

I've looked into other things that could be causing performance and MPG issues, and one thing that kept coming up is ignition timing.
I do have quite a bit of engine vibration at idle and some while under load. I watched some videos from Scotty Kilmer and learned that the timing advance / ignition timing will cause vibration.
Using my BlueDriver, I gathered this data from a 5 minute drive on smooth road with cruise control set at 45 MPH. The timing advance seems to be all over the place (see graph below).
Based on what I understand Scotty to be saying, the timing shouldn't be bouncing around like this at a constant RPM.

So what would be causing this variance in timing? Spark plugs? Distributor? O2 sensors? Spark plugs, distributor cap/rotor, and ignition wires were all changed last year (2020). I don't suspect those to be an issue, but I guess I should check. What would I be looking for in terms of excessive wear and how would I prevent it? O2 sesnors are still factory, so i suspect those are just old and need replacing. The second graph below shows the voltages for one of the upstreams, and it doesn't seem as consistent as I would expect. I understand O2 sensor data should be fluctuating up and down between 0 and 1, but the line graph doesn't move all the way down or up before trending in the opposite direction, so I doesn't have full confidence these factory O2 sensors are operating at 100%. I'm no sure if this data is normal or not. I'm hoping someone with more experience looking at this can tell me what's going on so I'm not having to throw parts at this, test drive, and hope that the data changes for the better. I should also point out that long term fuel trims are running within +5.00% now and no longer at the -13.00% it was a few months ago. I guess the fuel additive I've been using resolved that.

 

I guess that this is a 2nd gen Blazer which should be OBD II.

As you can read data you should also have access to all fault codes.
The OBD II is constantly checking if the CAT is still performing within specs. If not it will illuminate the "CHECK ENGINE" light and set a fault code. This is done by the second O2 sensor after the cat.

We have seen too many instances of rattling exhaust which finally was one or two of the heat shields.
Before venturing into major replacements I would definitively lift the truck and check the whole exhaust system.
Also exhaust leaks can throw fuel injection off.

Once it is clear that you have no codes but the fuel trims are off we can further investigate.

What would be interesting is to pull the following data exactly the same way you did for:

• O2 in front of CAT
• O2 after the CAT
• ignition advance
• fuel injection pulse length (if accessible - or any other fuel quantity)
• TPS
• MAF (if equipped)
• MAP (if accessible and/or available)
• Fuel trim (short term and long term)

Maybe George will chime in as well. Anybody else, other ideas to investigate without spending money yet?

 

Cats die in one of two ways so let’s build on Christine’s comments.

First, the catalyst is exhausted and the removal of pollutants is below spec. This is usually caused by a contaminated exhaust stream such as a rich mixture. That’s why all engine performance issues need to be straightened out before replacing an exhausted cat so that the inferior after market replacement is not also exhausted in a few months. This is determined by comparing the the upstream and downstream O2 sensor graphs and performing an oxygen storage test. This alone will not cause poor acceleration, just make the hole in the ozone layer bigger. Second is a mechanical restriction from a disintegrating honeycomb matrix. This will cause poor acceleration and is confirmed with a back pressure gauge in an O2 sensor port.

A single fuel trim number is of no diagnostic value. Please read my tech article in my signature and report all 4 values. Trends also have value but again, all 4 numbers.

You have control over CMP retard which is adjusted by rotating the distributor. The engine computer decides overall timing. Measure CMP retard and report back. Les can help you get this capability on the cheap (ish).

That O2 sensor graph looks OK at the voltage limits but I cannot asses the switching frequency or rise and fall time. Play with the horizontal scale and look for around 1 hz at idle and 100-200 ms switching time.

what is your actual mpg - do the math of gallons consumed vs miles driven when your at the pump. A poll here shows 17 mpg as the most commonly reported value which is what I get.

Subjective comments like lack of power can be difficult to address. There are some measurements but they are not the basic ones normally discussed here. Volumetric efficiency, load calculations, fuel delivery at full load, etc. I’m game but most don’t have that much fun off in the weeds with me. Short of the geeky world of engine performance measurements you can do a proper survey of all sensor value in live data as error404 has suggested. If you want to go there let me know and we can run down that rabbit hole and see what we can find. It’s lonely down here but interesting if you are so inclined.

Any time you have engine performance problems you always start with codes, fuel pressure, spark and sensor values. It’s free, it’s easy and you find a lot of problems that way. If nothing is found and your talking an older engine like this then Les would tell us to do a compression and leakdown test, especially with a complaint of poor power.


George

 

George, I think Blue Driver does CMP retard on IPhones. Not sure what they are calling it though. As I understand his scanner should be pretty capable, but no bidirectional controls. Please carry on.

 

I have a lot more data from that 5 minute drive at 45 MPH. It's just a real pain to make a graph of each endpoint from the CSV file BlueDriver exports.
https://docs.google.com/spreadsheets...f=true&sd=true

I'll look into measuring CMP, though my current concern is the timing advance values bouncing around like this at a steady RPM. Would that be CMP related?


18 MPG is average over 38k miles I arrived at for about 2 years in 2018 - 2019 when I was recording miles and gallons. It's all in a spreadsheet I can share. https://docs.google.com/spreadsheets...it?usp=sharing

This was when I wanted to know the true MPG and the fuel gauge wasn't getting correct data from the fuel sending unit. I replaced the fuel pump in late 2018.
Once the fuel gauge was reporting correctly, I try to refuel at 50% capacity. For my Blazer that would be 9.5 gallons remaining given the 19 gallon tank. At least I try to refuel it around that value.
Knowing what the true fuel level is at any time is a huge estimate. The fuel gauge isn't very accurate and I don't know if I fueled up to 19 gallons or perhaps 20 gallons. I only know how much I put in, not how much is remaining.
For the sake of my data, I rely on the auto-shut-off at the pump and determine that to be 19 gallons in the tank.

The lowest point in my recorded MPG is 14.6 and the highest being 27.7. Of course these are outliers of the over all data. On a bad day, the numbers seem closer to 17, but on a good day the numbers are closer to 20. I do a lot of highway driving. Lately It's been feeling more like 12 or 13 MPG. I'm barely hitting 130 miles before the fuel gauge reaches the halfway point.

 

Red Valor, FYI 1 data point every 1.3 sec can be problematic when trying to track quickly oscillating data like fuel trims and o2 sensor voltages. I always got about 30 data points per second back from the PCM on my BAFX Bluetooth OBD2 adapter to be split among the monitored sensors, and I would limit plots to only 2 or 3 sensors at a time. Not sure what you Blue Driver device is capable of for data points per second.

 

Yeah, I don't know why it's setup this way. I can't find a way to change the polling rate, so I guess I'm just stuck with it. This behavior makes it really difficult to work with when you want to track all fuel trims and O2 sensors. The BlueDriver app itself claims you'll get better quality data logging no more than 5 sources at a time. I only picked up the BlueDriver because it seemed like the best inexpensive option at the time that can read system modules for fault codes because I've been battling ABS issues for a while now and I still can't find the problem.

I really want to find a Scotty Kilmer or Eric from South Main Auto who can do advanced diagnostics without blowing big bucks on the diagnostic tools myself. I did discover a privately owned family shop in my area that has amazing reviews, but they misdiagnosed a drivetrain humming noise as rear wheel bearings, so not sure if I can trust them diagnosing engine performance that hasn't tripped any hard codes. I did get free wheels bearings from the place though. Still have the humming noise which I'm now convinced it's either transmission or front differential. I've been looking at an audio transmitter kit on Amazon to diagnose chasis noise to figure out what the noise is once and for all, but I'm not reayd to drop $300 on that just yet.

 

Probably the only way to increase the polling rate on an individual sensor or two is to uncheck the rest of the PIDs that are selected in "Data Sources". I always found a plot of RPM during acceleration/deceleration to be indicative of good polling...... smoth curve with no steps=fast data rate.