i have a 2001 4wd blazer and lately its been giving me crazy engine temp readings on the dash. typically it will warm up (and in the first minute or so will hold idle fine, but if i give it throttle, it will hit about 1500 rpms and then bog down heavy and start missing like a bitch) and after the temp needle starts to move, this issue will typically not affect the rest of the drive. But sometimes (less frequently cruising at highway speeds, more frequently right after cruising on the highway and going through stop and go downtown) the engine temp will shoot up to like 240, then back to around 200, then down to 100, 200, 160, 240 etc... sometimes the needle will jump multiple times per second. when this is happening, the idle will almost stall, shoot to 1500 rpms, drop to normal around 600, and dart back and forth (temp drop seems to cause idle spike and vice versa). i replaced the ect sensor and there has been no change. i did some research and a couple simple things to cross off the list would seem to be to "burp" the coolant system, and make sure the stat isnt stuck open (although it doesnt seem likely to me). it throws codes for lean banks, maf, and a few others. tomorrow i will get the exact codes and get any potential bubbles out of the coolant and post an update. i did some research that leaves me to believe that the air box check valve could be a part of the issue im experiencing. any insight or troubleshooting assistance would be very much appreciated.

 

*air pump check valve

 

Are any other gauges or functions fluctuating that fast? Your engine temp cant actually change that fast so its either voltage, wiring, connectors, sensors, or the ecm. Take a look at the system voltage when all of this is going on.

George

 

the voltage will bounce between battery voltage and alternator voltage with the bucking of the motor, similar slight deviations in oil pressure. but honestly this seems to be normal given the circumstances.

 

So when does it run smooth and when do it run crappy?

get the codes as you said but also look at live data and see if a misfire counter shows specific cylinders or P0300.

get a voltmeter and see if the voltage is always at least battery voltage when it runs bad, the dash meter may be too slow.


George

 

so i got a few bubbles out of the radiator and double checked the connector to the ect and noticed a collapsed vac line. i replaced it and scanned for codes and came up with the following (get comfortable):
-p0101
-p0118
-p0131
-p0151
-p0171
-p0300
-p0756
and the following codes pending:
-p0118
-p0131
-p0151
-p0171
-p0174

i just took it for a test run and the coolant guage seemed to be on point the entire time, no violent fluctuation in throttle like before. but on the way back i gave it aggressive throttle coming on to a highway and all of a sudden it starts running awful with very little throttle response, missing, and even backfiring. i hit 60 mph in about 2 minutes with it just about floored. very weak output and operating way over the powerband and seems to only respond full throttle (with weak output) or no throttle and nothing in between.

 

I guess you don’t like your truck very much. That’s enough abuse to burn a hole through a piston if it was running lean.

Good luck with your “diagnosis”

 

One possibility is a bad O2 sensor(s) but both banks report lean and the misfires are random so that's not my first guess.

When you had the problem before with code 118, the truck did not run great when warm so the ECT is not your only issue. You have a MAF code and a lean condition so you need to look for a vacuum leak in all of the vacuum hoses and look for unmetered air entering the intake after the MAF sensor. Also, turn the truck off, disconnect the MAF and then go for a run and see what happens.

Is there a difference in how it runs between just started cold and after it warms up? The O2 sensors are not involved when cold.

Can you look at live data and see what the O2 sensors look like in graph form?

Can you look at the TPS live and make sure its smooth key on engine off through the whole range?

George

 

i dont have access to any data like that. when it is running cold start, it acts very similar to when it happens intermittently getting on or off the highway. it will idle seemingly fine, but giving it throttle (unless feathered ever so slightly) will cause it to start missing bad and even backfiring. i replaced the o2 sensors and the maf or atleast cleaned it. i think most of those codes are the result of a similar issue. somewhere between the exhaust/intake exchange and/or the pcm. maybe egr valve/air pump check valve. i should definitely do a vaccuum test seeing as one line was visibly bad today. i have manually applied throttle from under the hood and everything felt smooth mechanically, but thats as far as ive checked.

 

If there is no difference between cold and warm then its not O2 sensors.

If you have vacuum leaks that must be fixed.

Don't forget the experiment with the MAF unplugged.

If you want to do your own diagnosis and repair then you need scanning capability that includes live data ans graphs with a few special features like CMP retard. The guys here have really good scanner solutions on the cheap. Member Les is a moderator here and one of the experts.

George