'98 Blazer - Codes, codes, codes! MAF & O2

[LAN2UNOW]

Well after hopefully troubleshooting things, it appears my gremlins are back... After replacing the MAF, the B1S1 O2 Sensor, the Air Charge Sensor and a general review by a mechanic on my Blazer w/about 110K on the sucker, we thought we had this licked.

It essentially would wait until it's brutally cold, like 17F or lower, then when you get up to freeway speeds within 5 minutes or so it damn near kills itself. Won't stall, but if you're at 65 MPH within a 1/4 mile it's down to 10 MPH. Pull off, let it sit for a few minutes and restart, you can drive it up to 50 MPH on surface streets. The Check Engine Soon light comes on popping codes along with it to the point we picked up a reader to attempt to troubleshoot these on the fly. Was stopping in at the auto parts stores to have them do it, but that got old real quick! Each time we've had essentially the same set of codes pop:

P0151: O2 Circuit Low Voltage (Bank 2, Sensor 1)
P0102: Mass or Volume Air Flow Circuit Low Input

Now there's also been 'lean mixes' reported in the past, but those went away after the MAF replacement and the O2 change. But the problem came back, we went after the Air Charge as well.

Originally we confirmed the MAF problem (see another post around here) by using the 'unplug' test. Unplug the sucker, if the problem goes away - bad MAF. Since replacement, the plug test does nothing - reasonably confirming that's been eliminated.

I've inspected the wiring on all the sensors I can easily get to, none seems to have any problems, cuts, etc. that could indicate it's simply a wiring fix required. While not a total troubleshoot, it reasonably removes that as a potential cause.

Only other reference I've found related to these codes popping, something the mechanic also mentioned, was coolant temperature sensors also creating the problem that'd trip these other codes and cause this condition. It was also mentioned that there's 2 of those sensors, located in two different spots and that they're not the same sensor as the O2's would be. Meaning one or both of'em gotta go. One's easy to get to, the other supposedly not but I'm trying to get some images that'd show where they both are to see if I can get it or if it heads back into the shop. Trying to avoid the shop until it's something I can't do myself like the MAF, Air Charge and some of the O2's even.

Anyone with any experience on this problem? Pics of the coolant sensors to be replaced, things like that?

Thanks!

Dave

[old_master]

98 has only one temperature sensor. It sends data to the ECM and the ECM uses the data for the engine controls and also sends data via class II communications to the dash gauge.

I see a whole lot of stuff being done but no mention of ever testing the fuel pressure and leakdown rate. Those are two of the easiest things to check and can cause so many different symptoms. Fuel delivery and secondary ignition are not directly monitored by the OBD system, but they can affect other systems that CAN set a DTC. The P0102 and P0151 could both be set as a result of low fuel pressure.

The P0102 means there is low input for MAF sensor, so disconnecting the MAF is not a conclussive test. Disconnecting it will set the same code. In other words, the ECM is receiving low voltage from it, disconnecting it will also produce a low voltage.

[LAN2UNOW]

Hmmm, I'll have to check on the coolant sensors then as I was told there are two on the '98 and all the parts lookups seem to indicate the same thing as they have 2 different items with 2 different locations...

Yes, there's a whole lot going on without fuel pressure testing simply because I'm DIY'ing and and most of us schmucks can't handle those tests without the proper equipment. So neither of those tests have been performed just yet, once it gets to that stage then it's mechanic time and he does all that. If it's something else I can fix before that to save some costs, I'm going to take a whack at it. Gas is bad enough, labor rates are worse.

The MAF test was never said to be 100% foolproof, but it does help determine if the thing is malfunctioning or not. When disconnected, the vehicle returned to normal operation - but with codes still tripped as you said. Plug it back in, the vehicle malfunctions again. The MAF that was in there turned out to be a remanufactured one that meant someone prior to me had replaced it already - possibly trying to debug the same thing.

Now you mentioned the fuel pressure, that's the first time I've heard that and it could be a vital clue! The pump/sending unit has been an issue, makes a whining noise and likes to give us fits at times. It needs to be replaced, but that's a $400 part alone, save dropping the tank, etc. that's beyond my capabilities or patience quite honestly. BUT, if it's causing those faults and it's freezing up or jamming up - maybe from fuel line freeze-up thanks to lovely reformulated Midwesten fuels - that could be the ultimate answer!

So thanks for that info, ain't the best I could hope for but if it's time to do it then it heads for the shop for some real answers - but at least I'll be armed with some sensible stuff to ask about and make sure it's properly checked.

[83cutlass]

Yeah there is 2 coolant temp sensors locations although both aren't necessarily used on all years. One is located in the left cylinder head and the other is in front of the thermostat. On a '98 I believe only one is used as old_master stated and it is the left cylinder one. There should just be a pipe plug in the T-stat location.