2001 2DR Blazer, 2nd owner, knew the first owner an history of it. 220k mile,well maintained,body and interior excellent condition. Has a random issue that I can’t fix. All of the sudden the trans will down/upshift in commanded, if at a red light it will fight shifting gears, rpm’s will rapidly swing up an down. Turn blazer off and wait couple of minutes and it will 70% of the time drive normal for a few minutes but will eventually act up again. Sometimes it goes all day before it starts acting up. Here is what over the last 14 months I have replaced: Trans totally rebuilt, my trans guy said other than normal wear it really could have gone another 50k, new fuel pump, new speed sensor, just changed the original poppet fuel delivery to the new injectors at the head system, distributor cap and rotor, new plug wires yesterday an checked plugs. Just drove it about miles in city traffic and ran perfectly, stopped at a store for bout 20 minutes, cranked it up and rpm’s sho to 3600, fought it all the way back home. Also I have removed and cleaned every harness ground point I can fine. Anyone got anything on this. I’m at my wits end here! Oh least I forget I installed a new ECM too!

 

Please further explain this: "All of the sudden the trans will down/upshift in commanded".

and this: "if at a red light it will fight shifting gears"

so: "cranked it up and rpm’s sho to 3600, fought it all the way back home" means that the truck was stuck at 3600 rpm for the whole drive home?

Any codes? Can you read live data with a capable scanner?

George

 

Thank you for your interest, you are my best chance to clear this up. If I’m at cruising speed in overdrive it will down shift one, maybe two gears then try to upshift.

lf I’m city driving and it start acting up, from one red light to the next it will not want to shift out of first and rpm’s go to 3 or 4 k before I back off, then it will usually shift up to 2nd. While at the light the rpm can climb to 4k in gear then suddenly drop to normal range.

At times while in city driving the rpm’s will climb and fall depending if it is able to shift up, I normally have control of rpm’s unless stopped at a light and it climbs to 3500 or 4000 in gear with my foot on the brake.

only code I ever get is the dreaded P0300, the Service Engine Soon light will light up, sometimes stay on, sometimes flash 14 to 20 times and repeat over and over, this always happens while driving at speed and running normal till it starts acting up. Last week after I replaced the spyder system I drove approx. 40 miles, city driving for about six miles, interstate for 8 miles then 4 lane the rest of the way at 60/65. Drove like a champ,excellent throttle response. After about 6 hours I started back, ran great for 16 mile till I had to stop to turn on the 4 lane and at that point it started it thing, I had to stop an restart it 3 times to finally get home.

 

OK, we have a lot of moving parts to this problem. We are going to need to break this down and just walk through all the possibilities. You are going to need a scanner capability that can read codes, look at live data (sensor values), graph O2 sensors, read fuel trims, read cam retard, etc. This can be done very cost effectively with at most $150 for a stand alone scanner or for as little as tens of dollars with a BT dongle and a smart device. If you have FU money then a high end scanner like a snap on is even better but thats not necessary. Here is a tutorial:

https://blazerforum.com/forum/2nd-ge...lazers-103424/

In the meantime, we need to figure out where the unmetered air is coming from that causes such violent rpm surges at idle:

Vacuum leak
IAC valve
Intake manifold gaskets
EGR valve
etc

I would start with making sure that your vacuum lines and brake booster are all in tip top shape. Fuel trims from your scanner will help diagnose and confirm repair if needed. Let me know if you need help with a vacuum leak expedition.

George

 

My neighbor has a Inova 3100 and I believe it has live data so I will get up with him today.

When I had the upper plastic plumne off for the spyder change I looked the lower intake gasket area over pretty good and checked the toque on those bolts, all looked well. Brake booster vacuum line is good as well as the other smaller vacuum lines in that area. I did have the IAC and cleaned it but not the EGR valve.

If I ca get the scanner today what data do you need? I can give you my cellular to call if you need real time data. On a whim I picked up a new coil Saturday but have not installed it. Distributor was removed about 2 months ago to check the gear and bushings, it is not the original one and checked good. I live in the Montgomery Al area, I was a WC 130 H and then a J model crew chief most of my adult life and only had one time that my plane had a electrical issue that took us weeks to hunt down but eventually we got it so I know this is a fixable issue.

 

The first thing is codes and freeze frame data. Then look at is fuel trims, long term and short term both banks. Then we need to look at some sensor values to see if they make sense, so to speak. Start with ECT, IAT and map on a cold engine. Then watch tps as you move the gas pedal slowly back and forth. If everything looks good there then we move on.

George

 

I did the reading on different scanners and I think the Vivant 400 best suits my needs, I have two GMs and one Toyota so I can buy the Toyota file when the time comes, I like the size of the screen an that it’s a bi-directional tool.

I will collect the data points you stated as soon as I can this week, just had a Level 5 plumbing problem show it butt this afternoon that demands attention first this week 🤬😡🤯.

 

Okay I have the live data.
Cold Eng-
ECT-24
IAT-24
MAP-46
MAF-8.32. RPMs were constantly bouncing, 630 to 850. This was both warm an cold.
TPS working smoothly.
Normal Temp, I’m listing all the points this Inova 3100 captures.
Fuel Sys #1/#2 in CL
Calc Load (%) 3.1
ECT. 75
STFT B1 Jumping 0.0 to 3.1
LTFT B1 steady at -3.9
STFT B2 Jumping 0.0 to 5.5
LTFT B2 steady at -8.6
MAP 35 to 36
Spark Adv. 16.5 to 33.0 I assume to changing RPMs
IAC 27
MAF 5.77
O2S B1 S1 0.050 to 0.785
STFT B1 S1 0.8 to 3.1
02S B1 S2 0.075 to 0.090

No DTCs active or in history. No Service Eng Soon or Check Eng lights during this run.

 

First, thank you for your service.

An over riding theme for all of this is making sure all connectors and wiring are sound with good connections that don’t go intermittent with a vibrating engine. You can move the connections and wires around while watching live data.

Look for any sensor reported value that changes with the engine surge that is not supposed to.

Another thing we can check is the consistency of your sensor reference voltages. Throughout the truck there are carefully controlled 12v and 5v ref and their computer grounds. We will come back to this later if needed.

Thanks for the data, this will help us. We need to see if the sensors values make sense and don’t surge with the rpm unless they are supposed to. ECT for instance will not change with surging rpm but some others might depending on the cause for the surge like map or MAF. I was not clear on cold engine, I meant cold and Not started, just turned to run. Let’s see what we know so far:

ECT: Three things could go wrong here, wiring, the sensor, air in the cooling system. If it was 24C that day and the reading did not fluctuate with rpm surge then all of this is probably ok. How high does ECT get when the engine is completely warmed up. 75 looks low.

IAT: same as ECT.

MAP: Needs to be redone with engine cold and not running and compared to ambient pressure that day. Manifold pressure will vary with rpm but also a variable vacuum leak

MAF: Looks high at idle. With engine off, unplug the MAF and restart. Does the problem get better? If the truck will let you, with MAF plugged back in, capture the MAF reading at a 1-2 full throttle shift.

Fuel trims: Your LT trims show that the engine normally runs somewhat rich and the the computer usually takes some fuel away to please the O2 sensors but not at a level of concern for an older vehicle. Your short term trims go from no additional compensation to adding fuel to account for what is probably unmetered air (air after the MAF sensor).

Spark advance: I don’t know offhand if this much spark advance change makes sense for those idle conditions. If we get stuck I will reproduce your conditions on my truck to see what I get but I am not at that location today which is also where my shop manuals are. Let’s ignore this for now but we will come back to it if we get stuck. Any history with the distr or cam sensor? Can you monitor CMP retard?

IAC: I have never had the IAC off of this truck so I don’t know if it moves the throttle plate or adds air through a port. If the former and its misbehaving then you would see the TPS value surge. If the later then it could be causing grief on its own. Never tried this before on this engine but unplug it and if it will idle, see how it runs.

Can you graph the two pre cat O2 sensors? We need to make sure that they are not causing havoc although they control fuel, not air. Any history of exhaust leaks or restrictions?


I still think that you have fluctuating unmetered air. Sources:

vacuum hoses, try clamping off or plugging the vacuum lines to the havac, 4x4, reservoir, brake boost, pvc, etc

have you tried spraying something like brake cleaner around leaking areas like the intake manifold gasket?

IAC as discussed

another possibility is a failing evac purge valve that becomes a leak source into the manifold. Any history here?

This should keep us busy for now.


George

 

I connected a vac gauge to the AC control hose this morning, held steady at 17.5. I also sprayed carb cleaner all around the intake but there was no change in rpm’s which this time remained pretty much steady. I pulled the distributor weeks before to check the bushings an the gear, no slack and no abnormal gear wear, 1st owner told me it was a replacement. Exhaust sounds and feels normal and the original CAT still installed, would 02 sensor behind CAT show a clogged CAT? I have felt every vac hose and replaced any I doubted, and will recheck. I will plug/unplug the sensors you mentioned to see what effects if any.
I need to drive it again to see how it does. I will get the cold readings with engine not running too.