Late model mechanics opinion

[mopar head]

Somewhat..
I`m pretty ignorant on modern computer codes for engine issues.
Vehicle is a 2000 gmc 3500 with the 7.4, a scan tool says egr valve and mass airflow sensor.
Are these 100% accurate? meaning if these parts were replaced the service engine light would go off? backstory-
Engine bucking at low speeds, gets better when warmed up.
At first I`d thought I`d check/clean plugs-good shape.
Replaced cap and rotor, plug wires. disconnect battery to rid service light. Drive, ran allot better, day later light back on. Still same symptoms on cold start/driving.
Stopped at autozone and had scan tool hooked up with codes mentioned above.
I`d just hate to buy expensive parts and not have that solve the issue.
Thanks guys for input:happy1:
btw the floorboard got allot hotter than normal, egr valve I assume?

 

[MOPARMITCH]

Do yourself a favor and Google the Year, Make, Model, and Engine and the specific codes on the interweb. I guarantee you will find lots of answers and diagnosis on various forums. You will get a pretty good idea where to go from there.

 

[67Dart273]

Computer readouts, as with any diagnostics are only a guide, "clues" and a direction to get you headed.

 

[toolmanmike]

Computer readouts, as with any diagnostics are only a guide, "clues" and a direction to get you headed.

Yes, just sensors that have gone out of their operating range. Techs plug in the scan tool first and then diagnose from there. It might need a lab scope on the MAF or a multimeter test of the egr. There is a lot of messing with connectors and grounds too. Most of the circuitry nowdays are "can bus'' where the computer controls the ground side of the circuit. One corroded pin in a electrical connector and something doesn't work. You can buy sensors and components but you can't return them if they don't fix the problem. Your choice to see if you can fix it yourself or hire it done. tmm

 

[mopar head]

Thanks guys, just the answers I was expecting.
I inspected the MAS, and noticed it looked pretty clean with exception of a few little black specks on the small bars that stick out in the airstream, I sprayed with mas cleaner to no avail.
All the connectors look real clean.
Oh well, maybe I`ll drop off at the stealership and bendover:evil3:

 

[mech1nxh]

Thanks guys, just the answers I was expecting.
I inspected the MAS, and noticed it looked pretty clean with exception of a few little black specks on the small bars that stick out in the airstream, I sprayed with mas cleaner to no avail.
All the connectors look real clean.
Oh well, maybe I`ll drop off at the stealership and bendover:evil3:

Pics of Freeze Frame Data???

 

[AJ/FormS]

If it took a day for the mil light to come back on, you might be in good shape,especially since the engine does in fact run. Surging and bucking can often be traced to the EGR valve. But the valve may be reacting to pressure in the exhaust. There are basically two types of EGR valves. The kind most of us grew up with since the late 70s, and the newer kind which are a feedback type. Since you were alert enough to notice heat in the floor board under your feet, Ima gonna go out on a limb and say that you have a serious exhaust restriction. The exhaust pressure is reeking havoc with the Egr feedback signal. Since it runs poorly when cold, I suspect the valve may also be stuck partly open, or more likely the pressure has blown the signal line off, or the heat has melted the rubber line.If the latter is true, that would explain the code for the MAP sensor. If I am right, full-throttle power will be down, and the heat in the floor will be up when the engine is called to work hard. I would highly recommend to measure the exhaust pressure to prove this diagnoses, before replacing parts.Feed-back EGR systems are not cheap. But then neither are Cats.
Do not run this vehicle before too much time passes or the exhaust valves will be burned.
The pressure test is easy.I think you should be able to tap into the exhaust system right at the EGR valve,feedback nipple.Run the test hose up to the windshield area, and clamp the gauge to the windshield with the wiper,then go for a boot.
Pressure over 4 psi means trouble. 2 or 3 seems normal. 3 to 4 would be borderline. You need to make the engine work, to create the pressure. If it builds pressure at idle, something is wrong,period. Pull the cat

 

[toolmanmike]

We used to squeeze the electronic box on the MAF or tap it with a screwdriver handle. If the engine quits or studders, it needs one. Just a old mechanics tip. I used to diagnose a lot of them at the service desk.

 

[Breadburner]

Could be O2 sensors.....

 

[eestatic]

Disconnecting the battery won't clear codes on OBD2 GM 'puters alone iirc, but w/battery disconnected hold down on horn button for ~30 sec or so to drain capacitor holding memory power to computer.

Not an expert by no means but to me it sounds like cat is plugging up.

Does it have a "muffled" sound when trying to open throttle up all the way and/or do you get a subdued back fire miss under load w/full throttle?

 

[mopar head]

Pics of Freeze Frame Data???

would autozone scan tool provide this? If so I`ll bug em again and take a pic if it would help diagnosis.

If it took a day for the mil light to come back on, you might be in good shape,especially since the engine does in fact run. Surging and bucking can often be traced to the EGR valve. But the valve may be reacting to pressure in the exhaust. There are basically two types of EGR valves. The kind most of us grew up with since the late 70s, and the newer kind which are a feedback type. Since you were alert enough to notice heat in the floor board under your feet, Ima gonna go out on a limb and say that you have a serious exhaust restriction. The exhaust pressure is reeking havoc with the Egr feedback signal. Since it runs poorly when cold, I suspect the valve may also be stuck partly open, or more likely the pressure has blown the signal line off, or the heat has melted the rubber line.If the latter is true, that would explain the code for the MAP sensor. If I am right, full-throttle power will be down, and the heat in the floor will be up when the engine is called to work hard. I would highly recommend to measure the exhaust pressure to prove this diagnoses, before replacing parts.Feed-back EGR systems are not cheap. But then neither are Cats.
Do not run this vehicle before too much time passes or the exhaust valves will be burned.
The pressure test is easy.I think you should be able to tap into the exhaust system right at the EGR valve,feedback nipple.Run the test hose up to the windshield area, and clamp the gauge to the windshield with the wiper,then go for a boot.
Pressure over 4 psi means trouble. 2 or 3 seems normal. 3 to 4 would be borderline. You need to make the engine work, to create the pressure. If it builds pressure at idle, something is wrong,period. Pull the cat

I assume actual exhaust gas goes up through this valve right? then rubber hose would not cut it for test right? The truck seems to have not lost any power and it carries a ton on its back full time.

We used to squeeze the electronic box on the MAF or tap it with a screwdriver handle. If the engine quits or studders, it needs one. Just a old mechanics tip. I used to diagnose a lot of them at the service desk.

Thanks I`m pretty good at tapping and beating on things lol

Could be O2 sensors.....

noted..

Disconnecting the battery won't clear codes on OBD2 GM 'puters alone iirc, but w/battery disconnected hold down on horn button for ~30 sec or so to drain capacitor holding memory power to computer.

Not an expert by no means but to me it sounds like cat is plugging up.

Does it have a "muffled" sound when trying to open throttle up all the way and/or do you get a subdued back fire miss under load w/full throttle?

I`d have to try a full throttle hit to see. I tend to baby this ole boy being it`s my $$$ maker. Like as said bucking only occurs when cold, I don`t seem to notice it at all after it warms up, runs pretty strong considering over 150.000 miles. I just like my junk not to run like----well,---- junkale:

 

[Abodybomber]

Pics of Freeze Frame Data???

You already know this... A freebie code scan, isn't gonna get you an actual useful tool...
Not your 39.95 Sun/Equis scanner setup.. The money spent on tools here, is well spent.... Freeze Frame/"real time " data,is what I used as a parts guy,to sell the better ( mid range stuff,200-300 dollar stuff..) equipment to heavy DIY'ers/ lower end shops.

 

[bcschief]

What were the actual codes ?

Brian

 

[ramenth]

would autozone scan tool provide this? If so I`ll bug em again and take a pic if it would help diagnosis.

I scan tool can provide this data. The Auto Zone scan tool can provide this data. Can the minimum wage parts slinger at Auto Zone find this data and know how to interpret it? Auto Zone actively seeks out people who don't know **** about cars so they can sell more parts to DIY'ers who will continue to throw parts at the car to "fix" it. They aren't techs, so don't get your hopes up.

As for the MAF... it may look clean, but if there are any burnt spots on the thermister it's toast. It takes a pen light and a good eye to see it. It should also being pulling around 8 grams a seconds at idle. If not then you need to take a look at underlying causes before you throw in a new one.

The EGR is a cold idle device. I'd suspect that. It can also throw off manifold vacuum, which could cause the MAF code through the MAP sensor. But does that mean it's the EGR? Not necessarily. Yes, they can be gummed up through natural running and if it's original with a lot of miles, then it's suspect. But, if memory serves me correctly, in 2000 GM used an external control very similar to a DPFE on Fords. Take a look at that, too. (And I'm not talking to see that it looks okay, I'm talking about actually testing it out.)

The thing of it is, there's no magical cure to today's cars. A million different things can cause just one code (okay, the number is exaggerated, but still) and someone with the right tools and the right knowledge can interpret the data correctly, do the test out correctly, eliminate the 99,999 things that isn't causing the issue, and fix it by honing in on just the one thing that needs to be fixed.

Don't look at the dealership. I can guarantee you that there's a shop in the area who have guys who can do the diagnostic work needed without the price of the dealership. I used to be one of those shops. Ask around and talk your friends, your co-workers, you clients, whatever, and find that shop.