98 tahoe MAF sensor?

I have a 98 tahoe with a 350 (5.7L) engine. It starts to run rough after it warms up. Through the years I have changed most of the sensors and stuff -- except the MAF sensor. There is no problem with the injectors or fuel pressure or anything like that. So I wanted to see what would happen if the MAF sensor was unplugged. I unplugged it and nothing happened. The engine ran the same - from idle and various rpms. I then shut the engine off and waited a while (with the MAF still unplugged) I then turned the key and the engine started right up and ran the same...Does anyone think the MAF sensor is bad? and can anyone suggest a proven test proceedure to easily diagnosis a bad MAF sensor?

I have a 98 tahoe with a 350 (5.7L) engine. It starts to run rough after it warms up. Through the years I have changed most of the sensors and stuff -- except the MAF sensor. There is no problem with the injectors or fuel pressure or anything like that. So I wanted to see what would happen if the MAF sensor was unplugged. I unplugged it and nothing happened. The engine ran the same - from idle and various rpms. I then shut the engine off and waited a while (with the MAF still unplugged) I then turned the key and the engine started right up and ran the same...Does anyone think the MAF sensor is bad? and can anyone suggest a proven test proceedure to easily diagnosis a bad MAF sensor?

To much sensor changing.
Sensors should be checked with.
A engine capable scanner shows the MAF sensor readings.
And if very bad it will set a code for MAF sensor.
Always check them for proper readings.
It reads Barometric pressure in HG and KPA and HZ.
The unplug test is not a good way to check them.
The VCM will substitute a fixed value for the readings.
A good lab scope will also read out what a MAF sensor is doing.

The biggest problem MAF sensors wiring plug in and dirt and oil getting in the sensor.
Also over oiled K & N filters and and dirty or cheap air filters.
If a MAF sensor is not reading right.
I always clean sensor/sensors with Maf sensor cleaner and re test it.

Well, I hooked up a scan tool that can monitor different things. The MAF sensor seems to be working properly. However, the O2 sensors are not. There are 4 sensors two pre- and two post- cat. I monitored the O2 sensor voltage output after closed loop operation and this is what I found. Bank 1 sensor 1-and bank 2 sensor 1 (I am assuming these are the pre cat sensors) actively switch voltage up and down. Bank 1 sensor 2 and bank 2 sensor 2 are acting differently. B1S2 acts like the pre cat sensors - but B2S2 switches the voltage for a few seconds and then seems to die - flat line. Can anyone tell me which of the two post cat sensors is working properly? And do you think it is a bad sensor or a converter or fuel problem? All 4 sensors have only about 15000 miles on them. They were changed at the same time along with the cat converters (two converters one unit). Thanks for the help...BTW - there is a check engine light and the code is P0302 "misfire on cylinder two":banghead:

Well, I hooked up a scan tool that can monitor different things. The MAF sensor seems to be working properly. However, the O2 sensors are not. There are 4 sensors two pre- and two post- cat. I monitored the O2 sensor voltage output after closed loop operation and this is what I found. Bank 1 sensor 1-and bank 2 sensor 1 (I am assuming these are the pre cat sensors) actively switch voltage up and down. Bank 1 sensor 2 and bank 2 sensor 2 are acting differently. B1S2 acts like the pre cat sensors - but B2S2 switches the voltage for a few seconds and then seems to die - flat line. Can anyone tell me which of the two post cat sensors is working properly? And do you think it is a bad sensor or a converter or fuel problem? All 4 sensors have only about 15000 miles on them. They were changed at the same time along with the cat converters (two converters one unit). Thanks for the help...BTW - there is a check engine light and the code is P0302 "misfire on cylinder two":banghead:

If the sensor flat lines there is a problem.
But.
The missfire problems needs fixed
first.
HERE IS SOME INFO ON O2 SENSOR TESTING.

http://www.troublecodes.net/articles/catfailure/

DTC P0420, P0421, and P0430 & P0431: Check Possible Cause Of Misfire DTC P0420 and P0421 indicate bank one catalyst system efficiency is minimum requirement. DTC P0430 and P0430 indicate bank 2-catalyst system efficiency is minimum requirement. Possible causes are as follows: Use of leaded fuel. Oil contamination. Cylinder misfire. Fuel pressure too high. HO2S sensor improperly connected. Damaged exhaust system component. Faulty ECT sensor. Faulty HO2S. Ensure ignition timing is correct. Retrieve all Continuous Memory DTCs. If misfire code is not present, go to next step. If misfire code is present, isolate cylinder and repair as necessary. Check HO2S Monitor DTCs If DTCs P0136, P0138, P0140, P0141, P0156, P0158, P0160, or P0161 were present in step 1), service as necessary before continuing. If none of these codes are present in step 1), go to next step. Check ECT Sensor DTCs If DTCs P0117, P0118, P0125 or P1117 were present in step 1), service as necessary before continuing. If none of these codes are present in step 1), go to next step. If any codes except P0420, P0421, P0430 and/or P0430 were present in step 1), service as necessary before continuing. If no codes except P0420 and/or P0430 were present in step 1), go to next step. Check Rear HO2S Wiring Harness Turn ignition off. Ensure HO2S wiring harness is correctly routed and connectors are tight. Repair or replace as necessary. If wiring harness and connectors are okay, go to next step. Check Fuel Pressure Turn ignition off. Release fuel pressure. Install fuel pressure gauge. Start engine and allow to idle. Note fuel pressure gauge reading. Increase engine speed to 2500 RPM and maintain for one minute. For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, go to CIRCUIT TEST HC. Check For Exhaust System Leaks If exhaust system leaks, it may cause catalyst monitor efficiency test to fail. Inspect exhaust system for cracks, loose connections or punctures. Repair or replace as necessary. If exhaust system is okay, go to next step. Check For Exhaust System Restrictions Inspect exhaust system for collapsed areas, dents or excessive bending. Repair or replace as necessary. If exhaust system is okay, go to next step. Check Manifold Vacuum Install tachometer. Connect vacuum gauge to intake manifold vacuum source. Start engine and raise engine speed to 2000 RPM. Manifold vacuum should rise to more than 16 in. Hg. If manifold vacuum is okay, go to next step. If manifold vacuum is low, go to step 11). Leave tachometer and vacuum gauge connected. Start engine and raise engine speed to 2000 RPM. On a non- restricted system, manifold vacuum should quickly rise to normal range as increased RPM is maintained. On a restricted system, manifold vacuum will slowly rise to normal range as increased RPM is maintained. If manifold vacuum is okay, no indication of exhaust leak or restriction has been detected and testing is complete. If manifold vacuum is low or slow to respond, go to next step. Leave tachometer and vacuum gauge connected. Remove exhaust pipe from exhaust manifold. Start engine and raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is downstream from exhaust manifold. Reconnect exhaust pipe to exhaust manifold and go to next step. If manifold vacuum is still low or slow to respond, fault is in exhaust manifold or intake manifold gasket. Repair or replace as necessary and repeat QUICK TEST. Leave tachometer and vacuum gauge connected. Disconnect muffler/tailpipe assembly from rear of catalytic converter. Start engine and raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is in muffler/tailpipe assembly. Repair or replace as necessary and test-drive vehicle to verify elimination of symptom. If manifold vacuum is still not okay, fault is in catalytic converter. Repair or replace as necessary. Check tailpipe/muffler assembly for debris from catalytic converter. Test drive vehicle to verify elimination of symptom.

based on the total amount of info given I would say the cat has been damaged....

I would clear all DTC's then re test for new failure codes...
you should have more than a miss fire code ...


as a way to bypass conventional testing and determine the cat's working ..when on cold start up go under the vehicle and with your hand feel that both cat's heat up the same...if one's hot and the other not, the cold cat is the bad one..

how many miles on the O2 sensors ? how many on this vehicle ?

There is 200000 miles on the odometer but the entire upper part of the engine is new - including the heads and everything else. Compression is good on all cylinders. There is 15000 miles on the O2 sensors..

I switched the two post cat sensors and there is no change in the performance, so I don't think it's the sensors.

The misfire issue -- I tested the coil (which I replaced a year ago) and according to the manual there are 3 tests. The only test which is not in spec is the - first connector pin to the distributor coil wire. The spec is 5000 to 25000 ohms. My test indicated the coil has 64000 ohms. Is that OK?

The DTCs -- after I tested the coil I checked the DTCs (for the 1000th time lol). I did get a P1351 -- ignition coil high input but I assume it may be because I was messing with the coil. Also, there was the usual code P0302 -- misfire cylinder 2. I reset them and drove for a while and the P0302 came back but there was also a P0300 -- which is a general misfire...So I checked the Distributer cap and rotor -- no issues.
I then observed the acutal spark for cylinder two and I noticed that the spark was not steady. It just stoped firing sometimes. I then observed the spark from cylinder 4 and it did the samething, but it was not as noticalbe as cylinder 2...I guess my issue is with the ignition system. What do you think would cause this condition? I replaced the ignition control module about a year ago...The wires(A/C Delco) are new and the plugs(A/C Delco Platinum) are new. The distributer is a new billet Accel distributor.

Things to check.
Fuel pressure up to par.
60/66 lbs pressure.
Good Hot spark out of coil.
Camshaft retard setting as close to 0 degrees - or + as you can get it.

If losing spark make sure coil has good hot blue spark at coil.
Check or switch plug wires.
Install a AC delco cap and rotor.

Clear codes and see if any come back.

The fuel pressure is 52 psi. When I rev the engine a little it goes up to 58 psi and then return to 52 psi. The pressure never dips below 52 psi (like when I rev the engine). I just put in a new fuel filter and no change. I changed the stinkin' fuel tank (because it was leaking) along with the fuel pump about a year ago. This is starting to bring back a few nightmares and headaches I had doing that job...
How can I test the fuel pressure regulator? It is located under the upper intake manifold.

The coil spark is good but it not consistant not totally blue like it should be....We're gettin somewhere---I am looking at my wiring diagram and I see that the 12v feed that feeds injector 2-8-6-7-and 5, also feeds the 12v feed to the coil as well as the 12v feed for the crank sensor. As I recall a few years back I had a problem with the crank sensors' wire -- it burned on the exhaust manifold. I fixed it, but maybe it has become corroded or damage. I'll check it after the Daytona 500 tomarrow. Do you think that if it is corroded or damaged that it could put enough voltage drop on the circuit to cause a misfire condition??

My scan tool is not capable of setting the cam retard spec, but the dealer set that when I had the crank sensor relearn proceedure done..He said it is at +/- 4 degrees..I think the spec is +/- 6 degrees..There has never been a DTC that indicates that the timing is off..

Your fuel pressure is to low.
Low fuel pressure will cause missfire.
You need 60/66.

Camshaft retart should be 0 degrees not over 1-2 at the most.

Only use a AC delco cap and rotor.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold and fuel pressure testing guide line.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

There is 200000 miles on the odometer but the entire upper part of the engine is new - including the heads and everything else. Compression is good on all cylinders. There is 15000 miles on the O2 sensors..

I switched the two post cat sensors and there is no change in the performance, so I don't think it's the sensors.

The misfire issue -- I tested the coil (which I replaced a year ago) and according to the manual there are 3 tests. The only test which is not in spec is the - first connector pin to the distributor coil wire. The spec is 5000 to 25000 ohms. My test indicated the coil has 64000 ohms. Is that OK?

The DTCs -- after I tested the coil I checked the DTCs (for the 1000th time lol). I did get a P1351 -- ignition coil high input but I assume it may be because I was messing with the coil. Also, there was the usual code P0302 -- misfire cylinder 2. I reset them and drove for a while and the P0302 came back but there was also a P0300 -- which is a general misfire...So I checked the Distributer cap and rotor -- no issues.
I then observed the acutal spark for cylinder two and I noticed that the spark was not steady. It just stoped firing sometimes. I then observed the spark from cylinder 4 and it did the samething, but it was not as noticalbe as cylinder 2...I guess my issue is with the ignition system. What do you think would cause this condition? I replaced the ignition control module about a year ago...The wires(A/C Delco) are new and the plugs(A/C Delco Platinum) are new. The distributer is a new billet Accel distributor.

as also mentioned and as I stated before the cat may very well be restricted...the O2 sensors need be replaced at around 100,000mi..assuming the usual , these sensors probably have well over 180,000mi...then the engine had extensive work...this all would contribute to the restricted exhaust\cat failure and damage.

fuel pressures need be around 60psi...52 psi will also cause miss fires...if you used a fuel pump other than delphi this is very suspect as well...It is very possible you have more than one problem..this confuses the diagnostics ...but testing fuel pressure and exhaust retrictions should show what is not correct...

OK, I indeed have two or more problems. First - I looked at the crank shaft sensor wire I fixed a-while ago. It had some corrosion so I just rewired the sensor back into the harness (where the wires are good). The spark is steady and strong now..I guess there was a voltage drop in that circuit..
Second - I pinched off the return fuel line and the pressure stayed the same 52psi. "DAM-IT!". There is NO unusual or rapid leak down - refering to the fuel pressure. In 30 minutes - with the engine off - the pressure reduced to 45psi. So I guess the fuel pump is getting weak? Right?

The catalytic converters - O2 sensors - were changed when I replaced the heads around 15000 (fifteen thousand) miles ago. After I improved the quality of the spark I scanned the O2 sensors again. The post cat sensors are now functioning properly. They do not flat line and they do not rise and fall as rapidly as the pre cat converters do.

The cap and rotor, as well as the plugs -plug wires and ignition control module, are all A/C Delco brand..

The DTC P0300 and P0302 still come up but it is not as noticable.

The engine runs better now but it runs a little rough. I guess it is because of the low fuel pressure. So it's back to dropping the tank again and replacing the fuel pump - or maybe I'll just cut a "trap door" in just above the fuel tank so I can easily replace the pump when it fails again..

I cannot thank you enough for leading me down the right path with this problem. The information I received on this site is great.. To think I started this with a question looking at the MAF sensor kinda embarasses me. If you can think of something other than the fuel pump causing my pressure issue let me know. I will post back in a few days when I change the pump -- AGAIN..Thanks-a-lot....

Do not forget to check that camshaft retard setting on a good scanner.
If fuel pump is bad best to drop the tank to replace it.
And only get a AC-Delco/Delphi fuel pump.
Airtex and after market pumps ar plain junk.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold and fuel pressure testing guide line.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

I have another question. Will a fuel pump for a 97 Tahoe fit in my 98 Tahoe? I mean the actual pump itself - not the whole assembly.
I do have a new ($free$) Airtex pump for my Tahoe but I don't want to put it in because the one that is in there has failed already. The pressure spec on a 96 and 97 is the same for my 98 -- 60 to 66psi. The only difference seems to be that you have to buy the whole fuel pump module starting in 1998. In 1997 the pump itself is avalible -- and a Delphi pump is around $80.00 bucks for a 97 - verses $270.00 for a 98.
I have disassembled the pump module and the actual pump looks just like the pumps I see in the pictures..Is there any difference that you know of?

If it is a airtex fuel drop it in the trash can.
You need to go with the complete fuel pump module in Delphi brand.
And one for your car year and model and engine.
Good Luck

I have another question. Will a fuel pump for a 97 Tahoe fit in my 98 Tahoe? I mean the actual pump itself - not the whole assembly.
I do have a new ($free$) Airtex pump for my Tahoe but I don't want to put it in because the one that is in there has failed already. The pressure spec on a 96 and 97 is the same for my 98 -- 60 to 66psi. The only difference seems to be that you have to buy the whole fuel pump module starting in 1998. In 1997 the pump itself is avalible -- and a Delphi pump is around $80.00 bucks for a 97 - verses $270.00 for a 98.
I have disassembled the pump module and the actual pump looks just like the pumps I see in the pictures..Is there any difference that you know of?

so I know I'm probably too late but check for the correct voltage at pump...it could be corroded connections/grounds or even the fuel relay...

this is an old vehicle the fuel tank/pump ground on frame rail...