99 Tahoe with a problem

[Bobby Bass]

I have a 99 Tahoe with the 5.7 engine. I get the engine light and two codes po300(misfire) and MIL (what ever that is) come on when I run a scan tool. I also will get a po175 code (mix to rich) once in a great while. I am totally baffled why this keeps coming up, Truck runs fine after we change out a part. (cold) but on the test ride the longer the ride the more it starts to misfire and run rough. There is still the smell of fuel in the exhaust and there was actually some dripping from tail pipe but disappeared after I cleaned the mass air sensor.

I have changed oil, cleaned mass air sensor, replaced air filter, replaced fuel filter, did plugs, wires, distributor cap and rotor, replaced coil. Each little thing makes it run a little better (178k) so it needed the mant. parts but still runs like *#%$@... Any ideas?

No codes for o2 sensor have come up and I don't want to start throwing parts at it. So far everything has been tune up and needed. But.......

[Jeremy airjer W]

I have to wonder if the bank 1 O2 sensor is not stuck rich. Should be pretty easy to figure out looking at scan tool data. If the O2 sensor is stuck rich it will cause a lean misfire.

If it is stuck and verified with scan tool data unplug it. The value should drop to around 400mv and stay there. Take it for a drive. If it runs better replace it.

[Bobby Bass]

Update, engine was scanned and it showed 6 of the eight cylinders had misfires, talk is now that it may be a bad spider injector but a bad cam sensor was also discussed, not a happy camper..

[Jeremy airjer W]

Thg CPI unit is plausible. Here is what I would suspect if it was the problem. The CPI unit delivers fuel. If it is delivering to much than the fuel trims will be way negative. From what I have seen its well over (or under depending on how you look at it) -25 % . If the CPI Unit is not dleivering enough fuel the fuel trims will be well over +25%. The normal scenario that I have dealt with several times is they do not deliver enough fuel and the fuel trims go crazy high.

The important thing is to rule out all the other variables. Mainly the MAF sensor, which if it is not seeing enough air flow, will cause a lean condition. And the O2 sensors which typically do not go biased rich on both upstream sensors at the same time.

The upstream O2 sensors can read rich when the engine is actually running lean. How you ask? If the lean condition is severe enough it will cause a lean misfire. This would result in the incomplete combustion of the air fuel mixture which sends an awful lot of hydrocarbons into the exhaust. The more hydrocarbons the less oxygen and the O2 reads rich.

If the CPI unit is replaced it would recommend going with the mutiport conversion. This will eliminate the poppet or mechanical part of the injector and put the electronic part of the injector right next to the intake valves. This will eliminate the dreaded minimum fuel pressure to start syndrome.

Hope that helps.

[Dave]

My 2000 with the same motor has a random misfire, too. The latest was last Sunday when I pulled my boat up north. Seems to misfire more when under a load. Didn't misfire in over a month then it did it twice in two weeks; once in windy conditions and the other when pulling the boat. Service Engine Soon light comes on then goes back off within two days. If I do ever feel it misfire, I can feather the gas enough until it clears itself and the SES light doesn't come on. Frustrating symptom though.

[Bobby Bass]

Update: Problem solved. Ended up replacing the fuel injectors (Spider setup) the last gasp hope was that the cam sensor was the culprit and we replaced it and it cleared the codes for about fifteen miles and then they were back. Shopped for the best deal on parts and we replaced the injector at home along with gaskets and gave everything a good cleaning. Runs like new again and it has taken care of the hard starting which has been a problem for a long time. I should not have to do anything under the hood for awhile. Complete tune up and at 178k miles I replaced a lot of things that are probably due anyway. Replaced air filter, clean mass air sensor, spark plugs, wires, cap and rotor, coil, cam sensor, fuel filter and injectors. Oil change and flushed the radiator even changed a few radio stations worse thing was I lost out on my fall fishing as the Tahoe is my boat towing rig..