Can Clogged Cats Cause 171 174?

[JusTheUsual]

171 174codes. Truck don't like to rev above 3k-3,500 rpm and almost no power at those higher revs. Fuel pressure ok. Compression ok. No sign of oil in antifreeze and no antifreeze in oil. This is making no sense to me

 

[koda2000]

Those codes usually indicate a vacuum leak(s) causing a lean condition (air getting into the engine which is unmonitored by the MAF sensor). I've also seem them come up with clogged fuel filters and/or low fuel pressure.

 

[JusTheUsual]

This is definitely a very weird problem. I don't think 171 174 is caused by clogged cats but by other test procedures its hard to figure out what is causing codes and it not liking to rev above 3k-3,500 rpm with no power at those higher revs. If its being driven normal it acts normal in power but when you want more power/more revs it doesn't go above those revs also no power increase at those revs. I know those 2 codes do usually indicate unmonitored air, like too much air getting into engine, but also thought maybe clogged cats would cause it not liking to rev maybe but why the 171 174 codes then. This is weird

 

[sgully67]

I agree with koda2000 I would look at fuel pressure. If fuel pressure is low it would explain the lack of power and the lean codes. Do you have the ability tom look at freeze frame data to see under what conditions it is throwing the codes?

 

[JusTheUsual]

Its not fuel pressure. Besides when fuel pump has low pressure or clogged filter or even clogged injectors it will still rev up like its suppose to just hardly any power. This wont rev above around 3,300 rpm and works fine with lower rpms but no power at about 3k rpms with no more revs. I was just thinking maybe cats clogged and with backpressure it wouldn't allow it to rev but I don't see how that would throw 171 174 codes. Also I have no sulfur or rotten eggs smell and never have so it doesn't make sense cats are gone/clogged.

 

[jimbalaya]

i have same codes

Those codes usually indicate a vacuum leak(s) causing a lean condition (air getting into the engine which is unmonitored by the MAF sensor). I've also seem them come up with clogged fuel filters and/or low fuel pressure.

I also recently got the CEL a few weeks ago and then bought a code reader, which indicates codes 171 and 174. I was about to post on my issue but saw this post.

I know from lots of reading on this website that those codes usually mean vacuum leak. what's the best way to diagnose the probable vacuum leak? I have heard about a 'smoke test' but don't know how to do one or if it's feasible for a backyard mechanic. Let me know any tips you guys have on this approach. the only symptom i am having from the two codes is a rough idle. otherwise the engine accelerates fine.

Also, to rule out fuel pressure how to I check that? I assume i need a special device? I know where the schrader-type valve is, by the way.

p.s. I had the same codes two years ago but turned it out to be the fuel filter, after cleaning the MAF and replacing the IAC. Since then I have replaced the spark plugs and wires as well.

 

[koda2000]

Other than a smoke test (might be worth paying someone to do one) it's really hard to find vacuum leaks. On the SOHC motor, intake manifold O-rings/gaskets are always suspect as is the PCV valve elbow.

 

[JusTheUsual]

I have a craftsman code reader. Its a nice one, not a basic one. Let me see if I can find link to show it. It does a lot more than just read codes.

http://www.sears.com/craftsman-canobd2-car-reader-trade/p-00914063000P?redirectType=SKIP_LEVEL

Right before this happened a few weeks ago the emissions test on it would read ready(The green light in middle of code reader). Now it reads red light which is not good or not ready and I haven't disconnected battery or anything. This is also besides the 171 and 174 codes its throwing now. Now I just checked again and its throwing 442. Wasn't there other day when I checked

 

[jimbalaya]

Other than a smoke test (might be worth paying someone to do one) it's really hard to find vacuum leaks. On the SOHC motor, intake manifold O-rings/gaskets are always suspect as is the PCV valve elbow.

thanks Koda, i will look into the smoke test. On my second question about fuel pressure, to rule that out, i assume i need a special gauge to measure the pressure?

 

[JusTheUsual]

thanks Koda, i will look into the smoke test. On my second question about fuel pressure, to rule that out, i assume i need a special gauge to measure the pressure?

Fuel pressure test gage. Screw it in Schrader valve. Turn key on check gage release pressure on gage. Start vehicle check gage. Shut off engine check gage

 

[bill06447]

Last 171/174 fault I had was a fuel pump. What are you getting for pressure readings? It's worth the $100 to buy the China freight scan tool; this lets you monitor fuel trim, O2 sensor voltage, etc so you may heaad down the right path when repairing the vehicle, rather than blindly throwing parts at it.

Bill

 

[JusTheUsual]

62psi but it has new fuel pump and tank also. When the pump was bad it still revved normal just like when the injector was bad it still revved normal just not normal power. This just don't like to rev above 3k rpm and no power at that range. I need to find out what causes a engine not liking to rev especially at that 3krpm and no power at that range. Anything below like 1k-2,500rpm the power is normal and engine acts fine. The only thing I am know coming up with is maybe the headgaskets are going but no oil in antifreeze and no antifreeze in oil which is weird because that's how you usually tell if head gaskets are done for

 

[2000StreetRod]

which intake system?

Does your engine have the variable intake system (VIS)? See: VIS Conversion

It was only used on 1997 and 1998 SOHC V6 engines but someone may have swapped your engine or intake manifold. Your 1999 electrical system does not support VIS which changes the length of the intake runners at 3,000 rpm.

 

[JusTheUsual]

nah mine is ohv 99 and engine hasn't been swapped

 

[JusTheUsual]

im starting to think more and more its gaskets letting in/ or maybe letting out too much air. Probably letting in to cause it to not rev above 3k rpm. I mean really, what could cause a engine to not let it rev above 3k rpm and have no power starting at those revs. The only new gaskets I haven't put on it recently is head gaskets. all other gaskets are new

 

[JusTheUsual]

well I just thought of something. Possibly the coil pack might not let it rev if no good spark. The recent plug exchange I did the plugs didn't look good and that was another reason why I thought maybe some antifreeze leaking in but no oil in antifreeze and no antifreeze in oil. The coil pack is less than a year old though. I got my old one and it wasn't totally shot so going to try that one to see if any difference here in it tonight. Not only that, if both banks 171 and 174 codes its not saying certain cylinders so could be whole coil pack and not like plugs or wires. But like I said before plugs and wires are new also with coil pack.

I just tried switching coils and still the same outcome. Right about 3,200 it don't like to rev but maybe like 200rpms above that but at 3,200 while revving in park or driving you can hear it stumble at that rpm and don't like to rev any more.

What about ordering and trying this? http://www.eachbuyer.com/mini-bluet...ource=Shopping_us&utm_medium=CSE&EachBuyer_us

That seems to tell a lot. Would this fit the obd2 for these trucks and anyone tried this or think it will help in diagnosing the problem

 

[ranger7ltr]

Probably not what you want to hear...

62psi but it has new fuel pump and tank also. When the pump was bad it still revved normal just like when the injector was bad it still revved normal just not normal power. This just don't like to rev above 3k rpm and no power at that range. I need to find out what causes a engine not liking to rev especially at that 3krpm and no power at that range. Anything below like 1k-2,500rpm the power is normal and engine acts fine. The only thing I am know coming up with is maybe the headgaskets are going but no oil in antifreeze and no antifreeze in oil which is weird because that's how you usually tell if head gaskets are done for

But if you are only getting 62 psi at the Schrader valve your fuel pump pressure is too low... Not very low but the specs are 65-72 psi...And I have the same vehicle you have and when the fuel pressure/volume dropped my truck would not pull upper rpm worth a darn but would idle just fine...I had a dirty fuel system with the tank rusty and dirty from fuel sitting in it for 2+ years...The codes you are getting are lean codes telling you that unmetered air is getting into the engine and the O2 system is seeing more air in the exhaust stream than expected hence the 171/174 codes...I am serious about the filter and the fuel pressure...I would remove the filter and see what comes out of it then try to blow air through it...

 

[JusTheUsual]

But if you are only getting 62 psi at the Schrader valve your fuel pump pressure is too low... Not very low but the specs are 65-72 psi...And I have the same vehicle you have and when the fuel pressure/volume dropped my truck would not pull upper rpm worth a darn but would idle just fine...I had a dirty fuel system with the tank rusty and dirty from fuel sitting in it for 2+ years...The codes you are getting are lean codes telling you that unmetered air is getting into the engine and the O2 system is seeing more air in the exhaust stream than expected hence the 171/174 codes...I am serious about the filter and the fuel pressure...I would remove the filter and see what comes out of it then try to blow air through it...

I've replaced the fuel pump twice in this vehicle in the 7years I've had it, also the tank twice. The fuel filter was replaced twice also. The last time I replaced pump, tank and filter was like a year ago. Both times I had to replace pump the pressure was at 35-39psi both times. It always revved fine even when I had a bad injector it revved fine. It would just not have the hp its suppose to have. This just don't like to rev over 3k but the lower revs it has power. The 62 is fine also the gage I got it leaks slightly and was fluctuating between 62-63 while slightly leaking gas out of bottom of gage. Its had that same 62psi when I replaced pump as it does now. So that should be the key factor in why its nothing to do with fuel. The lean codes or unmetered air as it sounds like it might be gaskets going or gone is what im starting to think. I put the new injector in 2months ago so its almost all new gaskets except intake manifold which is below the fuel manifold and head gaskets. Those are the only 2 well 3 because of 2 heads gaskets I haven't replaced recently. Also I know my number 3 cylinder has a cracked exhaust manifold gasket leakage plus I think the other side does too plus a small hole in exhaust right before cat under by drivers feet. I was told on here cold air or air leaking into the exhaust from the holes and cracked exhaust manifold gaskets can cause the lean codes because they hit the o2 sensors. So I had those codes for a few months but just recently in past couple weeks this rev thing happened. So my mind has been all over the place I actually didn't get title of this thread right. But I was thinking mayb cats clogged and mayb back pressure especially with holes in exhaust before it and cracked exhaust manifold gaskets but usually bad cats have a bad smell to them and they don't. Like I said, my mind is all over the place even now with all of this and I don't even know what to write next

 

[crunchie_frog]

If it were me, I would get a way to look at the actual live values of the fuel trims, O2 sensor readings, MAF reading. You can for sure diagnose if it is an air leak with a scan tool which lets you see all of these things, live, preferably on a chart. I am sure you know this, but the PCM looks at the MAF and then sets a preprogrammed amount of fuel by adjusting the injector pulse width. It then looks at the O2 sensor before the cats and it adds or subtracts from the programmed pulse width by adjusting the fuel trim. When the fuel trim gets about 25% away from the preprogramed value, you get the lean or rich codes. What you should see is your fuel trims maxed out and the PCM will not adjust more than so high to compensate. So you could have a dirty MAF, I am assuming you checked/cleaned it and you can verify it is working (again I think you would benefit from a scan tool that can see everything live). If you have an air leak, your fuel trims will be the greatest at idle and will go down as you go up in rpms. You can spray the combustible of your choice around your potential leak points and you can see the fuel trim cut back if you have a leak. To me, it sounds like fuel delivery or a bad MAF. As you go up in rpms, with an air leak, the fuel trims should go down when compared to idle.

I use FORSCAN and you can see live graphs of your fuel trim, O2 sensors, MAF flow, rpms, load, etc. I would start with something you can see what your PCM is seeing to help with troubleshooting.

If you are thinking it is plugged catalytic converters, you should be able to diagnose this with a vacuum gauge.

 

[JusTheUsual]

Thanks crunchie for that info(everyone else too). Nothing like exchange of ideas/opinions and brainstorming. I was just thinking it would make more sense for it to be the fuel for the issue because if it is too much air into engine why wouldn't it run ****ty at lower rpms and better at more revs. But just all the tests I've done shown fuel is fine so im sure you's see why I kind of elimated fuel but revert back to what I just said about air leakage it should run ****tier at lower rpms not higher...sigh, just cant win...

I know I should somehow see live whats going on but I have no way of doing that. Crunchie where is this forscan and how much and how to do it? Do you have link to it for this vehicle make and knowledge on how to use it? Can I use laptop instead of phone and sounds like I need to buy cables to hook it up.