failed emissions test
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failed emissions test
I have an 84 F150 with a 351W engine. The engine was replaced with a rebuilt one by the previous owner a couple of years ago, but I believe all the emissions equipment is all in the stock configuration.
I rebuilt the carb (Motorcraft 2150) last week because it had been running poorly. Now it idles smooth and runs great once it's warmed up. When it's cold, it stutters during light to medium acceleration, but picks up if I hit the gas harder. I think this is just the choke being poorly adjusted and I'm still working on that.
I took it in for an emissions test this morning. I made sure I drove it enough to get it good and hot first. The report shows no problem at idle, but high HC and CO during the driving test. The report sheet says this indicates incomplete combustion and/or mixture is too rich.
So what would cause rich mixture during driving but not at idle?
Actual test results:
HC: 15.4425 GPM (limit is 5)
CO: 343.7376 GPM (limit is 55)
I rebuilt the carb (Motorcraft 2150) last week because it had been running poorly. Now it idles smooth and runs great once it's warmed up. When it's cold, it stutters during light to medium acceleration, but picks up if I hit the gas harder. I think this is just the choke being poorly adjusted and I'm still working on that.
I took it in for an emissions test this morning. I made sure I drove it enough to get it good and hot first. The report shows no problem at idle, but high HC and CO during the driving test. The report sheet says this indicates incomplete combustion and/or mixture is too rich.
So what would cause rich mixture during driving but not at idle?
Actual test results:
HC: 15.4425 GPM (limit is 5)
CO: 343.7376 GPM (limit is 55)
Elder User
Joined: Nov 2004
Posts: 666
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Some causes for rich mixture during driving by not at idle (not in any particular order):
1. Float level too high. The idle mixture is governed mostly by the idle mixture screws, and is less sensitive to float level than mixture under load.
2. Someone (previous owner) decided to rejet the carb and put in larger main jets.
3. If this is a feedback carb, could be a bad O2 sensor. They can start to go bad, causing rich mixture under load long before the computer decides that it has failed and the check engine light comes on. The fuel control system runs open loop at idle, so a bad O2 sensor will not cause mixture problems at idle.
4. Fuel pressure too high. Not likely if it has the stock fuel pump.
There are probably others that I have not thought of. This will help you get started.
Good luck.
1. Float level too high. The idle mixture is governed mostly by the idle mixture screws, and is less sensitive to float level than mixture under load.
2. Someone (previous owner) decided to rejet the carb and put in larger main jets.
3. If this is a feedback carb, could be a bad O2 sensor. They can start to go bad, causing rich mixture under load long before the computer decides that it has failed and the check engine light comes on. The fuel control system runs open loop at idle, so a bad O2 sensor will not cause mixture problems at idle.
4. Fuel pressure too high. Not likely if it has the stock fuel pump.
There are probably others that I have not thought of. This will help you get started.
Good luck.
Last edited by f100jim; Jan 30, 2009 at 09:38 AM. Reason: Fixed typo.
Elder User
Joined: Nov 2004
Posts: 666
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Thought of some more. The first time, I was thinking strictly of fuel system problems.
Could also be an ignition problem.
1. Timing may not be advancing with RMP. Delayed spark could lead to imcomplete combustion. I once had a '66 Mustang where the previous owner had put 2 heavy springs in the distributor to cut back the advance. I think he was trying to use the "Sunoco Penny Pinching Pump" gas of the time. Car ran fine, but would overheat on the highway. Point is, the timing could not be advancing enough, but it would still run pretty well.
2. Fowled spark plugs. They may fire reliably at idle, but open the throttle and build some cylinder pressure, and it may start to misfire. You would probably notice this, though.
3. Bad spark plug wire(s) - same as #2.
I'll be back if I think of anything else :-)
Could also be an ignition problem.
1. Timing may not be advancing with RMP. Delayed spark could lead to imcomplete combustion. I once had a '66 Mustang where the previous owner had put 2 heavy springs in the distributor to cut back the advance. I think he was trying to use the "Sunoco Penny Pinching Pump" gas of the time. Car ran fine, but would overheat on the highway. Point is, the timing could not be advancing enough, but it would still run pretty well.
2. Fowled spark plugs. They may fire reliably at idle, but open the throttle and build some cylinder pressure, and it may start to misfire. You would probably notice this, though.
3. Bad spark plug wire(s) - same as #2.
I'll be back if I think of anything else :-)
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Does the egr work? That comes into play during driving. To see if it works, find a vacuum line or a un-used vacuum port that has vacuum at idle. Get a spare hose and plug it in this port and get the engine running, the spare line should be sucking air. Unplug the hose going to the egr and plug in the spare vacuum hose you hooked up. The engine should stall.
Then hook everything back up as original, get the engine fully warm, and then unplug the hose at the egr, and then hold your finger over the hose while you rev the engine. You should feel a slight vacuum as you rev the engine, and no vacuum when it's idling. If this all checks out, then the egr is working.
Then hook everything back up as original, get the engine fully warm, and then unplug the hose at the egr, and then hold your finger over the hose while you rev the engine. You should feel a slight vacuum as you rev the engine, and no vacuum when it's idling. If this all checks out, then the egr is working.
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Joined: Nov 2005
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I have an 84 F150 with a 351W engine. The engine was replaced with a rebuilt one by the previous owner a couple of years ago, but I believe all the emissions equipment is all in the stock configuration.
I rebuilt the carb (Motorcraft 2150) last week because it had been running poorly. Now it idles smooth and runs great once it's warmed up. When it's cold, it stutters during light to medium acceleration, but picks up if I hit the gas harder. I think this is just the choke being poorly adjusted and I'm still working on that.
I took it in for an emissions test this morning. I made sure I drove it enough to get it good and hot first. The report shows no problem at idle, but high HC and CO during the driving test. The report sheet says this indicates incomplete combustion and/or mixture is too rich.
So what would cause rich mixture during driving but not at idle?
Actual test results:
HC: 15.4425 GPM (limit is 5)
CO: 343.7376 GPM (limit is 55)
I rebuilt the carb (Motorcraft 2150) last week because it had been running poorly. Now it idles smooth and runs great once it's warmed up. When it's cold, it stutters during light to medium acceleration, but picks up if I hit the gas harder. I think this is just the choke being poorly adjusted and I'm still working on that.
I took it in for an emissions test this morning. I made sure I drove it enough to get it good and hot first. The report shows no problem at idle, but high HC and CO during the driving test. The report sheet says this indicates incomplete combustion and/or mixture is too rich.
So what would cause rich mixture during driving but not at idle?
Actual test results:
HC: 15.4425 GPM (limit is 5)
CO: 343.7376 GPM (limit is 55)
I had the very same thing happen to my '84 F250 4x4 with the 300 I6 + NP435 + NP208 last year. It runs excellent but will not pass emissions on both HC and CO.
My thought is that the 25 year old catalytic converter is shot and this is normal for one of these trucks when there is no cat. working.
Fortunately, vehicles this age no longer need to be tested where I live, so I don't worry too much about it anymore.
I have replaced the O2 sensor and the TPS and also tuned up the carb. This all helped but did not cure the high CO / HC problem.
The emissions tech told me that some of the cars he's tested show HC and CO in the hundreds or thousands of ppm, so I didn't feel too bad about being 40-50% above the low limits on HC and CO when others out there are 500-1000% over them. After trying to fix this, we all get the same variance, so what the hey.
My truck runs good and there is no visible smoke from the exhaust. It starts easily, is dependable, gets good gas mileage, and is paid for. If it washed itself, it would be perfect! ;-)
Ed
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I can see where a higher operating temp would be better for vaporizing the fuel and getting better combustion from that. Not sure what my thermostat rating is, as I haven't changed it. Might be worth checking, though.
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If the motor was replaced or rebuilt by the previous owner it may not be low enough in compression (8.2:1) in 84, or maybe too much cam. If the truck runs good I would run a line from the air pump directly to the catalytic converter, bypassing the thermactor valve thingy. This will make your cat run VERY hot and should burn off alot of extra CO and HCO, it should get you through emission testing but disconnect immediately after because your cats will be ruined if you run like this for too long. My local carb shop did this for me last year and got me right through. Also I read somewhere that people add some alcohol or something similar into their gas that seems to work well, search around a little you'll find it. Good luck. Joe
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If the motor was replaced or rebuilt by the previous owner it may not be low enough in compression (8.2:1) in 84, or maybe too much cam. If the truck runs good I would run a line from the air pump directly to the catalytic converter, bypassing the thermactor valve thingy. This will make your cat run VERY hot and should burn off alot of extra CO and HCO, it should get you through emission testing but disconnect immediately after because your cats will be ruined if you run like this for too long. My local carb shop did this for me last year and got me right through. Also I read somewhere that people add some alcohol or something similar into their gas that seems to work well, search around a little you'll find it. Good luck. Joe
Running the cat at a higher temp will definitely make it more effective at oxidizing the combustion by-products... IF there is any active catalyst remaining.
Remember that catalytic converters are only designed to operate for 5-7 years before they are replaced. Some will work for much longer than that depending on their construction and use.
As you say, running at elevated temps for an extended period of time is not a good idea, as that can certainly damage the catalyst substrate, which is likely to drastically reduce its efficiency.