Mystery Problem?

[L&L70]

I tried searching, but I'm having a tricky time even explaining it. I have an 87 swb with a TBI 305 and TH400. Its has complete tune up with plugs, wire, cap, rotor, fuel filter, ignition coil (went bad), ignition control module (tested bad) and perfect fuel pressure. As soon as you give it throttle it responds as expected and then immediately "bogs" down. It feels like the transmission is doing it. Its less noticeable after warming up. I do not know much about transmissions or what controls them. You can throttle out of it so to speak, but under normal throttle it feels like the converter is locking up? Sorry for the amateur hour, looking for ideas. Thanks

[Greenlee]

The th400 has a vacuum modulator mounted on the passenger side that controls shift points. It is cheap to replace. I would check that and the vacuum line running to it.

[chengny]

I doubt it's transmission related but if it feels like the transmission is up-shifting too soon (and bogging the engine down), bring it up manually. Start off in 1, let it bring your speed up to where it would normally make the 1-2 shift (like 5 mph). Without letting off the gas, kick the selector up to 2 and let it wind out to the normal 2-3 upshift point (maybe 15-20 mph) and then pop it into D.

If you don't get that sensation of the engine being overloaded by manually upshifting, then look into the modulator (esp. the vacuum line from the manifold - as suggested above) and the governor. However, at least in my experience, problems with either of these components generally result in delayed rather than early shift points.

[L&L70]

Never thought of that quick test. When operating cold the problem is the same in drive or manual shifting. Basically disappears when it gets up to operating temperature. I noticed the idle surges slightly while cold also.

[v30crewcab]

had a bad torque converter in a th400 do the same thing.

[L&L70]

V30, did it straighten out when it reached operating temp? I got to wondering if it had anything to do with my bad idle while cold. I'm figuring the idle issue is a sensor which would have no communication with the tranny. Truth is I don't really have a clue. That's why I'm here.

[jermasaurus]

You mentioned that the fuel pressure was perfect. Did you happen to monitor pressure as the truck is acting up?

You can have good pressure with key on/engine off, or at idle. But the pump may not be able to keep enough pressure when driving. Basically you may have pressure but not enough flow if that makes sense?

I've seen that a few times on the TBI trucks.

[L&L70]

I stood on the brakes and throttled up till the tire broke loose and the fuel pressure did not drop. I keep thinking the TPS, but I wouldn't think it would remedy itself after warming up.

[jetmech85]

There is some good stuff on throttle body trouble shooting in the 1988-1998 section of this message board.