Howdy all,

I just upgraded the exhaust system on my truck (flowed exhaust ports, long tube headers, 2.5 mandrel bends, 2.5 to 3.5 Y pipe, 3.5 cutout, 3.5 racing cat, and 3" muffler) I did all my math and sized everything to maximize flow for now and future upgrades. I also added plenty of 02 bungs for custom tuning down the road, so all the goodies are there like they should be.

problem i'm having now is that she is stubborn to start when cold (have to feather the gas 2 or 3 times), she won't burn rubber like she did before upgrades, and she can barely pull texas hill country hills at 40mph.

its almost like shes "banging out" at 80% to WOT, but still runs fairly well the rest of the time.

fuel pressure is in spec. base timing is set at 12*. new spark plugs gapped to .050 (a little wide i know). ignition module checks out fine.

what am i missing here??

 

forgot to mention, due to the headers, i no longer have a functional egr system or functional smog system.

the smog pump was re-plumbed immediately before the cat and blows 100% of the time.

any assistance is greatly appreciated... cause my head is starting to hurt!

 

The smog pump blowing air into the exhaust is your problem. It's changing the readings from the O2 sensors, this is making the computer do things to the fuel curves it shouldn't be doing. I'm surprised it has a smog pump. My 97 E150's 351 never had one to start with.

 

Well you now have an exhaust system big enough for a bigblock, problem is you still have a stock small block so exhaust air velocity had gone way down and taken all low rpm torque with it.

How to fix it? Get a LOT more air through the motor.. meaning replace the whole topend(HCI) with aftermarket parts and tune it, and/or add a supercharger.

 

thanks for the replies.

My truck being a heavy duty model retained a lot of old technology where the light duty ones didn't. the smog pump port is nearly 2 feet down stream of any 02 port, and wouldn't that give me a false lean reading, causing a rich reality? as it stands now she is actually leaning out on the big end.

I'm workin on checking the AFR with a standalone wideband 02 sometime today, so real numbers coming soon.

true enough the system can support nearly 600 lb/ft of torque (that was the idea), however from all the numbers i crunched the stock 351 should still be extremely happy. ultimately the truck will get a built stroker motor, so lots of money yet to spend on the top end. (it will definitely get a better intake when all is said and done)

It's almost as if she is having a fuel delivery issue, possibly unrelated to the exhaust upgrades?

again, thanks for the help!

 

Yes quite possible, a wideband O2 will definitely help you see if it's going lean but then you will need some way to monitor fuel pressure to see if that is the cause. A stock fuel pump should have no problem supplying this motor with fuel so something bigger isn't needed yet.. just one that is up to spec.

 

from previous testing, the fuel rail pressure is within specs on both tanks. I'm thinking that somehow i sucked up some trash and its clogging the top side of one or more injectors.

first order on the day, wideband o2
second, pull injectors and clean

will report my findings later on today/ tonight.

as always, thanks for the replies and help.

 

Well fuel pressure at idle vs under full load at rpm are two different things, I think if you got dirt into the injectors it would show up as a bad idle and stumble more than just poor power at speed. I think you need to setup a fuel pressure gauge you can see from the drivers seat even if only temporary.

 

Thanks for the detailed help here Conan, I do have a pressure Guage that is viable while driving and it was still in spec.

I'm thinking it might be only a partial blockage. though it's acquired a solid miss at idle now, and been finicky on cold start (80* out today, so more like cool rather than cold)

 

ok, so as expected my afr is typically 13.5 to stoich at idle unloaded.

but under unloaded acceleration, or any load at all she shoots as high as 20.- and has a prominent miss.

definitely fuel related! now to dig in and clean some injectors.

 

alright, update time.

pulled, cleaned, and reinstalled injectors.

major recovery of performance! still a slight miss noticeable.

afr at idle ~10.-, under partial load ~14.5, under full load ~16.6

 

So it's possible you have 2 problems.. dirty injectors and a weak pump. When you say "under full load" do you mean at higher rpm or still at relatively low rpm?

 

Sorry about that, I'm a diesel mechanic, so full load at rated power is what I meant. which for this motor would be roughly 3k rpm for max torque? (no tachometer in truck so I'm guessing, and diesels never spin fast enough to bother with horsepower)

from everything I'm experiencing and that the injectors don't show to have ever been replaced (305k miles) I'm going to try a fresh set of injectors and see where that gos... once the funds are available.