P0174 And Random Multiple Misfire

nhproxr · August 18, 2008

I am getting an occasional code for P0174 and random multiple cylinder misfire. Truck is a '98 GMC Yukon with vortec 350 with 170k on it. Thing is the ses light doesn't stay on, but it runs bad all the time. Back in January I changed all 4 O2 sensors and put a pair of new cats on.

Truck starts fine, and runs good cold. Once warmed up idles real rough, almost dying. I also notice engine pings off idle when you give it gas sometimes. Part throttle drives okay, slight surge from time to time. If you try to give it any throttle over 1/2 hesitates then blows black smoke and kinda goes....lacks a lot of power. Also at idle the exhaust smells horrible, seems very rich. Truck feels like something is plugged up or something holding back power. Back in Jan when I had codes for low voltage at O2 sensors, it ran bad, but when pushed it would come to life (and look like a Detroit Diesel out the tailpipe)....feels different his time.

This truck is still running with original intake gaskets, injectors, fuel regulator, etc. Only things ever done to motor are the 4 O2 sensors this past Jan '08, 2 pre-cat O2 sensors a year earlier (maybe something burning out sensors??), new cats Jan '08 and a new fuel pump years ago at 115k.

Since I have changed O2 sensors a couple times in a year and half or so is something ruining them? I hope its not an intake gasket cause I don't have time for that swap right now.

minibike132 · August 19, 2008

P0174 is lean on bank 2 so I would start checking that bank for an exhaust or vacuum leak. A vacuum leak on either side can create a code on only one side so pay attention to any place that could be leaking but for exhaust really only need to check bank 2. I don't suppose you have a scanner that can read cylinder missfires, fuel trim or O2 readings do you. If all you have is a code to go off I would check those things.

ChevyTech007 · August 19, 2008

Sounds like intake gaskets leaking. Here's how to check it. Take the PCV valve out of left valve cover and the tube off of the breather from the right valve cover. Start the engine and plug the tube and the hole in the left valve cover with your fingers and see if a vacuum develops. Any vacuum indicates intake gaskets leaking. It should build pressure slowly because of blow-by. I bet it builds a vacuum.

nhproxr · August 19, 2008

Sounds like intake gaskets leaking. Here's how to check it. Take the PCV valve out of left valve cover and the tube off of the breather from the right valve cover. Start the engine and plug the tube and the hole in the left valve cover with your fingers and see if a vacuum develops. Any vacuum indicates intake gaskets leaking. It should build pressure slowly because of blow-by. I bet it builds a vacuum.

Thanks I will try this test tomorrow. Will this tell me there is an intake leak only or there is a vacuum leak somewhere...like also possibly a hose or something?

nhproxr · August 19, 2008

P0174 is lean on bank 2 so I would start checking that bank for an exhaust or vacuum leak. A vacuum leak on either side can create a code on only one side so pay attention to any place that could be leaking but for exhaust really only need to check bank 2. I don't suppose you have a scanner that can read cylinder missfires, fuel trim or O2 readings do you. If all you have is a code to go off I would check those things.

Just a code reader.

nhproxr · August 19, 2008

Sounds like intake gaskets leaking. Here's how to check it. Take the PCV valve out of left valve cover and the tube off of the breather from the right valve cover. Start the engine and plug the tube and the hole in the left valve cover with your fingers and see if a vacuum develops. Any vacuum indicates intake gaskets leaking. It should build pressure slowly because of blow-by. I bet it builds a vacuum.

Okay, I did this test today and I definitely have pressure building. When I took my fingers off while running I could feel a little poof of air coming out, more so on the pcv side then the breather side.

Ceck engine came on today and stayed on. Throwing just P0174 now, no random multiple misfire.

What's next?

nhproxr · August 21, 2008

Anyone have any ideas?

I've looked the exhaust manifold over and doesn't appear to be leaking anywhere. Also I have checked all hoses for leaks or unplugged and everything looks as it should.

Since this is just a lean condition on bank 2, what things would affect just one side of engine. Seems as though most fuel issues can be elimnated as well as iginition cause that would affect the whole engine. A bad injector or 2 would throw a misfire on a specific cylinder right? MAF sensor would affect whole engine right?

Really scratching my head here. Should I just replace )2 sensor on that side and see what happens? They have a 1 yr warranty, and I just replaced them all in Jan. Maybe I could get a new bank 2 sensor 1 sensor.

nhproxr · August 26, 2008

Help! I'm still scratching my head at this one guys. Truck seems to run fine while warming up during open loop. Once it reaches temperature starts with the problem. Has a real rough idle, almost like it wants to stall. When pushed harder it sorta smooths out, but lacks some power and when really pushed it blows some black smoke out tailpipe. While cruising at light load it is constantly feeling its missfiring or something.

ziptow · August 26, 2008

I'd check again for a vacuum leak..it seems that the leaks happen on the bottom of hoses etc. I find the best way to locate vacuum leaks is to water spray bottle from the dollar store.

Get the vehicle warmed up and start spraying the hoses, gasket areas etc (watch the plug/coil wire...could give a jolt). If you have a large leak..you should be able to hear the ater being sucked in. A smaller leak will cause the engine idle speed to drop or the engine to jump around.

I know you have DTC's from one side of the engine..but it won't hurt to get some throttle body cleaner and hit the throttle plate. I like to remove the IAC valve, clean the tip, compress it and reinstall it. I would also check the fuel pressure before and after the filter.

ziptow · August 26, 2008

Message duplicated?

Bought a new laptop and it decides to post when it wants?

nhproxr · August 27, 2008

I'd check again for a vacuum leak..it seems that the leaks happen on the bottom of hoses etc. I find the best way to locate vacuum leaks is to water spray bottle from the dollar store.

Get the vehicle warmed up and start spraying the hoses, gasket areas etc (watch the plug/coil wire...could give a jolt). If you have a large leak..you should be able to hear the ater being sucked in. A smaller leak will cause the engine idle speed to drop or the engine to jump around.

I know you have DTC's from one side of the engine..but it won't hurt to get some throttle body cleaner and hit the throttle plate. I like to remove the IAC valve, clean the tip, compress it and reinstall it. I would also check the fuel pressure before and after the filter.

Thanks, I'll try these tips. I did try looking around for a vacuum leak with propane hoping for a jump in idle speed, but found nothing.

I was thinking about cleaning throttle body, just in case. Also I was gonna run some injection system cleaner (maybe techron...any suggestions of best brand?) thru it to see if that helps. Also been a bit since a new air filter and pcv valve....fuel filter was done last year, but was gonna throw one in anyways.

I drove some distance on 2 lane interstate yesterday at speeds around 55 and truck seems to smooth out a little and cel actually went out. Once I hit stop and go again it started idling rough and cel came right back on. Don't know what kind of clues this gives??

ziptow · August 29, 2008

Kinda sounds like water in the fuel considering you are getting faults on both sides of the engine? Get a fuel sample, look for water on the bottom and try some "water remover" and not fuel anti-freeze..big difference.

nhproxr · September 3, 2008

Kinda sounds like water in the fuel considering you are getting faults on both sides of the engine? Get a fuel sample, look for water on the bottom and try some "water remover" and not fuel anti-freeze..big difference.

I'm only getting the lean code on bank 2.

glock-31 · September 15, 2008

I am having the same problem. I have a 98 GMC Sierra. The code the mechanic said was Random Cylinder Missfire. About 3 weeks ago it started to idle rough, worse when in drive and stopped, and horrible when in reverse. It starts up every time perfect but after it warms up the rough idling starts. Check engine light came on and stayed on. Slower than 40 KM/H it has power problems but faster than that it seems to run fine. In the last week I noticed the battery is at 10 while idling, 14 while driving. Normally it is at 14. Engine temp is normal. I replaced the Air filter, Spark plugs, spark plug cables, PCV valve. Added engine cleaner (to oil), fuel injector cleaner, and etholeen glycol (gets rid of water in gas) to gas. Oil change was recent, fuel filter is ok, O2 sensors all fluctuating correctly. Distributor cap is only 8 months old. nhproxr, anything new with your truck? you get it running yet? Thanks for all info given so far, and any further that will be given.

Rob

95Sierra2500 · September 15, 2008

If you're getting random misfires, you may want to consider first doing a tune-up, and second having the injectors cleaned at the dealer. Have them run their Top Engine Cleaner through it. It's a VERY common issue for the poppet nozzles on these CMPFI systems to clog up and start sticking. If they are sticking closed, you'll be lean, if they're sticking open, you'll be rich. It sounds like yours could be sticking closed, which is good because they'll only have to run the Top Engine Cleaner. If they were sticking open, they'd have to get in there and replace the poppet nozzle, which usually means replacing the injector too.