Re: Howell TBI vs COLD weather
Just so everyone knows the logic behind it.
On a telephone conversation Roger mentioned a key factor. He was watching down the throat when it was doing it's stumbling.
---- (Please, please, please - whenever you do that wear safety goggles, all it takes is a backfire to permanently blind you.) ----
As he cracked the throttle the fuel shut off for a split second, it stumbled, then fuel started squirting again.
Ony a few thing happen just "off idle."
Hmmmm, TPS? No, he'd already swapped in another one. The likelyhood of two being bad at the same spot is -- awful!
MAP sensor? But it's not on a hard snap, just "ease into it off idle," a hard snap it's OK. The MAP sensor doesn't see enough vacuum drop on a soft "ease into it." It should also do it at higher speeds too, not just "off idle."
Could the Tach Sync signal to the ECM be getting lost for a split second? What would cause that? How does the engine know it's trying to accelerate? It gets it's RPM info from the tach input signal - from the module.
The module gets it's sync info from the pick-up coil.
The vacuum advance just barely moves at about 1200 RPM, right when the stumble occurs - could it be the PU coil wires themselves?
Tried something -- Disconnecting the vacuum advance made the cold stumble go away, proving it! I've seen the pickup coil wires go bad before, but it's a very uncommon problem. And never as a temperature related problem.
(Those tiny wires have to bend every time the vacuum advance moves - think about it - it's mind boggling how many times those tiny stranded wires have to move in 100,000 miles. Suprisingly, it's not a more prevelant problem.)
Just thought it was interesting how greenlighting this came up with the answer --- He He -- I HOPE!
Roger has had quite a few issues with the Howell swap.
Those contemplating it maybe can avoid a few by knowing what he went through.
- First he had a problem with the adapter for the throttle body. There's a big gap at the side -- Howell said plug it with silicone. Silicone "unsticks" with gasoline - causing a severe vacuum leak. The cure he did - there's a flat area on the adapter next to the hole. He simply used a blob of JB weld to extend the side of the adapter out a little to cover it. Used a file to make it flat on the bottom, and made a new larger gasket. Simple, easy.
Helped quite a bit.
- His HEI had the wrong vacuum applied to it - manifold vacuum suggested by Howell, instead of ported vacuum like an HEI distributor needs. Simple, hook it to the right vacuum source. Helped even more.
Since the HEI distributor was not from a TBI type vehicle, the ported vacuum from the TBI was not compatable with the distributor. Wrong curve, probably too fast coming in, too much too soon.
Using the correct vacuum source and re-curving the distributor correctly with an adjustable vacuum advance did the trick. All he needed was a timing light to do it.
Now the stumble, rough idle, tip in, ping, and other issues were gone. But still performance was not what it could be.
Then, disconnecting the vacuum line to the EGR helped even more. Roger said that alone made a huge difference in performance - now that the other issues were gone. Smoother and more power.
All the above - each trouble accentuated and masked the next one - it took addressing them one at a time. Sometimes we look for one simple answer instead of thinking multiples - fooling ourselves.
Everything worked great for a few months. But then it got cold outside. Only on very cold mornings did it stumble, 25 degrees or less, but as soon as the engine warmed up for a moment it no longer stumbled, ran fine the rest of the day. This didn't happen until recently, but was it? Since it never got that cold before who knows.
More chip burning from Howell. They've been very helpful and willing to try, but their focus of attention is on the computer aspect of it, not the entire engine.
All the first issues were due to the matching/mismatching of components from different vehicles. Two problems were introduced by Howell, vacuum source and silicone. They kept trying to overcome mechanical problems with chip burning.
Now this most recent one was actually from a defective part.
Hopefully this will save someone from headaches, or help someone improve performance on what they have now.
You have to look at the whole thing as a "system." ALL the individual components all work "together" to make it a "System."