MAF sensor replaced, idle still rough

chevrolet
colorado

#1

My 2008 Chevy Colorado started idling rough. I took it to Advanced Auto for a ECU read and it said that the MAF sensor was bad. I replaced it. Cost $98. Ouch! The check engine light did go off. But the idle is still rough. I checked for leaks using propane. Air filter, spark plugs, oil filter are all new. I poured Seafoam fuel additive and it did absolutely nothing. I pushed throttle in neutral–no effect. Engine mounts seem to be good. Only 47k miles. There are no noises other than when I step on the pedal on the road I can hear a howling, hallow noise… like wind in a tunnel. What do I check next?


#2

That is actually pretty cheap for any repair part let alone a MAF. Try cleaning the throttle body by pulling the intake hose off and spraying cleaner made for that purpose - I’d use carburetor cleaner.

BTW, Is the CEL back on?


#3

No. I put the new MAF sensor in and the cel went off. But idle did not change at all. Still rough


#4

Have you checked the air duct between the MAF sensor and the throttle body for cracks or splits? That would also explain the howling noise.


#5

Oh, yes. Unbolted, rebolted, checked with propane. The vehicle only has 47k miles. Mostly freeway. All the hoses feel solid.


#6

It didn’t say your MAF sensor was bad, It said your MAF sensor detected a problem.

Make sure the duct work between the air filter box and the intake manifold is intact, sealed and not cracked, including MAF sensor gasket. checking with propane won’t find that air leak.


#7

I would also entertain the idea of the dirty throttle plate.
Cleaned mine recently (for the first time, at 85K miles) and it improved idle a lot.


#8

Oh, OK. Anyway, I put in a new sensor and the check engine light went off. I will check that duct again. I already unbolted it once. It still looks new. Besides, I read that if there if a vacuum leak, the rpm will go up at idle. But the rpm is at 800 and steady. It was 750 before I changed the MAF sensor.


#9

another idea: check if ECU need to perform “MAF learn” procedure on your engine.

This is definitely the case on my Nissan and procedure is very simple to follow (essentially: warm up, stop, disconnect MAF, start the engine and let CEL to get on, re-connect MAF, clean code, let vehicle idle for 15 minutes not touching accelerator). It may be different for another car, but should be possible to find on Chevrolet-specific forums.


#10

Thanks. I will try that. I joined coloradofans.com forum and asked about the MAF learn. How do I clean code? Do I need to buy a scanner?


#11

Good ideas above. If they don’t pan out, ask your shop to check for diagnostic codes again, and let us know what the fuel trim measurements are. It sounds to me like it is idling too lean, which should show up in the fuel trims


#12

They told me they can’t check the codes again because after I replaced the MAF sensor the check engine light went off. They only check it if it is on.

I suspect that it could be I lean too. But I can’t find any vacuum leak. I cracked open the filter cover and nothing changed. I checked with propane.

I checked the resistance on the ignition coils and all 4 were the same. And they were all getting 12V. And I don’t see any bad hoses. All still firm. I can’t find the PCV valve though. Maybe there isn’t one. There is a hose under the intake housing that connects to the engine and it is firm. Fuel pump seems to be working. I disconnected the fuse to it and car would not start. Put it back, it started right up.

But maybe it is lean. I am thinking of buying an engine scanner myself. Is there a basic one that will graph the fuel trims?


#13

These guys are idiots, no doubt about it

Even with the check engine light off, you could have a stored code or a pending code. You could also check to see if certain readiness monitors have run to completion.

NEVER go to Advance Auto . . . or autozone, for that matter . . . for advice on car repairs or diagnosis. They are not mechanics and their advice is free. That should tell you something :thinking:

On this website, the advice is also free. But there are a lot of mechanics and extremely knowledgeable and experienced diy guys on here.

Only go to an auto parts store to buy auto parts, nothing else.


#14

Harbor Freight sells quite a few models. You might surf over to their website and take a peek. I don’t own any vehicles post 1992 model year, so I don’t need an OBD II scanner myself. But for fuel trim I think you need one that advertises “mode 6”, or “freeze frame”, or “real time data”, something wording like that. HF usually has a downloadable user manual for each product, so you can see exactly what each model does in detail by reading those.

On the 2.9L engine there isn’t a PCV valve per se, but there is a PCV orifice tube, which performs sort of the same function. It’s plugged into a hole at the top of what looks to me like the valve cover, and is connected via a short 90 degree hose to the intake manifold. There’s a fresh air inlet from the air cleaner resonator on the opposite side of the injectors. A plugged or leaking pcv system will result in rough idle.


#15

Try disconnecting the negative battery cable overnight. This will force the computer to reset to the factory defaults and relearn the parameters.


#16

I tried disconnecting the battery overnight I got throttle cleaner and sprayed a lot. I checked the ignition coils. All reading the same resistance and getting 12V. I could not check the spark because there is no way to take the coils out and connect the intake hose because the big intake hose runs right over the ignition coils. The engine has to run to check the sparks right?
I ordered a scanner that can graph short term and long term fuel trim. I can’t find a vacuum leak anywhere. Is there some spot I may have missed?


#17

That could turn out to be a big mistake.

Tester


#18

From the article;

In most cases, you should be fine to use carburetor cleaner to clean your throttle body.

However, be very careful. On newer cars or trucks, the throttle body flange is covered in a clear coat to improve air flow. Using carb cleaner could potentially remove this delicate coating. Another thing to keep in mind is that throttle body cleaner is safe on the electronic sensors where as carb cleaner is not.

I knew about the sensor issue but clear coat?? Waddya Know! Learn something new everyday. Thanks!


#19

Did you ever figure it out mine is doing exactly the same


#20

You should probably start your own thread

That said, can we assume you also have a 2008 Colorado . . . ?

Without even laying hands on your car, I’m going to say there’s a pretty strong possibility your engine is misfiring, and may be badly misfiring

ALL of the 4- and 5- cylinder engines for this truck were known for having top-end problems, which lead to low compression and severe misfires. This even happens with low-mileage engines