Hello all, it's been a while since I've posted around these parts as mainly I've just been driving the heck out of my car and enjoying it. However, I've got another project I've been working on, and it's just got me stumped.

The culprit in question is a TBI. I've got the car running great, and it performs pretty well and gets great gas mileage for what it is. However, on the down side, it's got a bobble in overdrive that I have not been able to solve.

It only happens when the engine is under a load (going up a hill or accelerating beyond a gradual pace), and it only happens when the Torque Converter is locked up. The car runs great up until the gas pedal hits a certain point, and then it feels like it's falling on its face (we're not talking rapid acceleration here, just the pressure applied to slide past somebody on the interstate or climb up anything more than a slight incline on the highway.) It continues to stumble the farther you go with the gas pedal until the transmission downshifts, and then goes away until it shifts back into overdrive. When its starts stumbling, you can drop it in drive, and the problem dissapears almost completely (it's still there, but almost impossible to feel when the engine is turning at the higher RPM).

Now, my original thought was the torque converter, but I chased this line down completely and it didn't fix the problem. I first installed a B&M Torque Converter Lock-Up Control kit since I had the unit laying around after I converted the other car to a 6-speed, thinking it might be the switch. It was then I found that if you turned the lock-up on the torque converter off, the engine would pull smoothly up any grade or passing condition, regardless of RPM or whether it was in 3rd or 4th gear. This led me to believe the Torque Converter was the issue, so I ordered a new B&M Torque Converter and Shift Kit (just the entry level performance stuff, no racing or high stall stuff). I also changed the fluid and filter (obviously) with Valvoline Dex/Merc High Mileage Synthetic Transmission fluid (Dexron VI grade). This made no noticeable impact on the jerking/bogging/stumbling at all. Thinking it might be the torque converter solenoid, I pulled the trans back apart again and replaced that with a new Borg Warner unit. Didn't help.

I'm stumped now. I took it on a trip through western Maryland and West Virginia last weekend, and almost drove myself crazy unlocking the torque converter via switch every time I came to a hill to keep it from lunging and jerking like crazy. Does anyone have a clue what might be the problem? I'm down to one of two ideas:

1. Something is messed up in the TBI. I hope not, because I don't have a clue where to start with that.

2. The engine is being lugged too much by the torque converter locking up, and I simply need to install a vacuum cutoff switch that unlocks the torque converter when I fall below, say, 10" of vacuum.

Any input? Anybody had a similar problem before? I appreciate the help, I'm starting to get a little frustrated.

Thanks,

-Levi

 

Forgot to post what all other modifications I'd made to the engine:

-MSD Ignition Coil
-Fresh Distributor cap and rotor
-New plug wires and Bosch Platinum Plugs
-Advanced timing by 4 degrees (set at 4 degrees BTDC w/ wire unplugged)
-Full tube headers w/ custom exhaust and dynomax muffler.

The only thing that could possibly be impacting my problem is the advanced timing. I would think that would make it better though, not worse. Also doesn't explain why it only does it with the torque converter locked.

 

It sounds like the timing. Set the timing at 0 where it belongs and see how it works then. Also check your fuel pressure.

 

Will definitely check the timing this weekend. I wouldn't think it would be the fuel pressure, as the problem would only get worse as I went further down with the throttle, not better.

 

Unplug and block the EGR hose to the EGR valve. See if your problem goes away. If it does, install an EGR valve for a TBI 305/350 truck. The stock L03 car EGR valve relies on exhaust backpressure to keep it closed. Truck EGR setups require backpressure to OPEN. You have taken the backpressure away from your exhaust system which allows your stock EGR valve to flop right open. That is the difference between positive and negative backpressure EGR valves.

 

I wouldn't set the timing to TDC even on a stock engine. 4-6* is where these engines tend to run best.

 

Still chasing my tail on this one. I moved the timing back to 0 degrees, no difference. I checked the voltage on the TPS, it's .63 at closed throttle and moves to 5V at WOT, with no glitches in-between that I can tell. I also switched the MAP out with one from another car that was running great, and this did not fix the problem either. I checked the EGR valve, wasn't that. Now I'm wondering, could this possibly be in the distributor? The car has 276,000 miles on it, and I doubt the distributor was ever changed when they did the motor. Could something worn out in the factory computer controlled dizzy be causing this problem? If it is, I've got the distributor out of my other car that I pulled when I converted to HEI laying around somewhere, I just don't want to go to the effort of swapping it out only to hit another dead end. Any other ideas?

Thanks,

-Levi

 

I'm having some somewhat similar problems but never really thought about it being a tranny issue. I couldn't even drive in OD it was so bad. I posted a thread about it and got some good info. RFmaster here on the board said that most aftermarket ignition coils don't work well with these dizzys. I noticed you had a MSD coil. After I replaced the cap/rotor and coil with a OE parts it took care of 90% of the problem I was having (for a day or so). I've got other unknown issues that are probably related to internal dizzy wear as well but it might be something to look at first if you have a known good OE coil laying around. They're about $20 new. Good luck, I totally know how frustrating it is, I cant catch my tail either..haha

 

Finally figured out where I left my spare dizzy. Of course it would be 2300 miles away on the garage shelf back home... Dad said he was going to drop it in the mail, so hopefully I'll be figuring things out here soon enough. I've got a few of the coils sitting around too, although I drove around on the MSD coil in my Firebird for months without a hitch. Then again, the transmission torque converter lock-up wasn't hooked up on it either...

I could be nearing a solution though. I guess I have to be, there's not that much stuff left to replace.

 

try putting your old ignition coil in. I had a weird issue less than 2 months after installing an msd. stumbling etc.

 

I've got one of the Accel Ignition Coils from my old TPI car that I wrecked stashed away somewhere. It's not the kind that looks like the factory coil, it's an older style that I guess they don't make anymore, a gray colored box that bolts to the firewall. I'm going to see if I can dig it up out of the storage shed this weekend and give it a try. If not, I'll try and find a factory coil around here somewhere, shouldn't be too hard. I'm also throwing in a new MAP sensor, ignition module, and distributor cap/rotor while I'm at it. The silly thing has 276,000 miles and I use it for a daily driver/long distance road trip vehicle, so some upkeep is necessary to make me feel comfortable with it, you know?

 

Dont forget to grease the plate on the ignition control mod. 276k ?! thats what I have on my Camaro, rock on high mileage.

 

Somebody educate me on the TBI truck EGR valve. I have a 92 Firebird 305 TBI that I have been fighting a stumble on the idle and poor higher RPM performance. The EGR valve is new, but for the LO3 car engine. I gutted the converter.......