TBI timing issue

[Russell]

Hey guys!

I'm trying to sort out a bog my TBI system I swapped into the 77 has right of idle.

I have to set the timing around 12 degrees BTDC with the timing mark unhooked to get any reasonable amount of power out of the engine in the top end, but that makes the idle timing like 30 odd degrees, which makes it spit out the exhaust at an idle, and really bog if you lightly push on the go pedal. If you drop the hammer, the thing lights em up, then roars off. The bog doesn't recover if you just hold the pedal slightly off ide, it just keeps bogging till you hit the pedal harder, or let go. It also surges 3 - 4 times really bad if you hit the go pedal hard, then let off to turn or something/

If I set the timing to 0 degrees, like it should be stock, it idles properly, but you can really tell that it is missing a whole lot of power. You can floor the thing, and it accelerates about the same as my old 4 cylinder buick century

The engine is a bone stock mid 80s 350, TBI is bone stock, with all new Delco sensors (MAP, 02, TPS, CTS, IAC), with 350 injectors (fuel pressure is fine), and a stock 350 prom.

I've already checked for vaccum leaks, and I'm fairly confident I don't have one. At an idle with the timing set at 0, I pull 22" of vaccum, but at idle with the timing set at 12, I pull about 12" of vaccum.

There is no such thing as a happy medium, unfortunately...

Is my only option to get an aftermarket chip for it? I'm not set up to burn the TBI style proms...

It seems that might be best, as I need more timing, and thats exactly what an aftermarket chip would do...

 

[HarryH3]

Do you know if the cam has ever been swapped in the engine? It sounds as if the cam timing isn't right. Perhaps it was installed one or two teeth off from TDC.

 

[Russell]

Didn't change the cam when I did the swap from carb.

I'm pretty sure it is something electrical, cause it has done this on two seperate 350s it's been installed on...

Originally the setup came on the 305 in my 89. I killed that engine, so I picked up a 73 350 shortblock, and got the heads, intake, knock sensor, EST module, chip, and injectors from from a TBI 350 a buddy of mine had disassembled in his shop. After installing it, it bogged like this all the time. I spent countless hours trying to figure out what was wrong with my scanner, and trying various other spare TBI components I had kicking around.

That engine started knocking, and I blew the transmission again, so I pulled the engine and tranmission, along with the whole TBI setup from the truck. I gave the truck away, scrapped the blown transmission, and tore the engine down to figure out what was hammering.

I then decided to drop the whole getup into my 77, which has an 84 350 "HO". This time I used the stock 84 heads, but swapped the TBI intake on by adjusting the center intake bolt angles. I then replaced all of the sensors / actuators with either known good stuff, or brand new stuff, just so I made sure it was all going to work properly in the end. Did the exact same thing on this engine as well...

My original thought was maybe the camshaft is the problem, just in needing a different timing curve in the prom, but I've heard of gobs of guys who've swapped a TBI setup onto their bone stock engine, cam and all, with excellent results, so I am not entirely sure about that anymore. The engine ran great with the carb, despite being a pain in the but to get started in the morning...

 

[HarryH3]

Hmm, well first ya said this:

The engine is a bone stock mid 80s 350,

Then ya said this:

I then decided to drop the whole getup into my 77, which has an 84 350 "HO".

The HO motor is WAY different than "bone stock".

Moneypit put an older HO 350 into his TBI'd truggy (The 300 HP version, I think it was?) and had to get a custom chip burned to get it to run decent. Even then he didn't like the lack of low RPM throttle response, due to the cam that GM put in the engine. It's really made for hot rods, not rock crawlers, and comes on STRONG from about 2,500 RPM and up. We swapped in a cam made more for low end torque and he's much happier. Just a blip of the throttle at idle now and that thing leaps.

 

[Russell]

300 horse in 84?

Nah, the thing ran glass smooth, and felt like a pretty normal small block to me when it was carbed... The reason why I put it in quotes is becuase I am not entirely sure that was the stock engine to the truck I pulled it out of, quite possible it isn't even an HO engine at all.

I could see it having problems if it was an early 70s HO, those things definitely had a lot more gusto than the mid 80s engines, and certainly had a far larger cam than EFI would be happy with, but it has done it on both 350s, one of which I know for sure should have been suited just fine to the EFI...

 

[Thunder]

TBI ECM will not work with any 350 HO without a custom chip. Cam is way out of specs for the Computer to handle. Will run lean and eventually burn a hole in your piston. (worst case scenario)
Need a custom chip. Not hard to find. Most chip burners have a program for the HO 350.
But swapping in a computer frendly cam is the best way. Way better low end.
Even the older stock 350s cam had more lobe separation than TBI ECM likes and wont run well on a stock chip. They were made for carbs not fuel injection.

 

[Russell]

Hmm, interesting... The thing usually runs rich though, until you hit like 4500 rpm, then it leans out.

However, thats just what the ECM is telling me, there is no telling exactly what is going on without a wideband...

I think I'll try a TBI cam swap, and see if that improves it any...

 

[southernspeed]

Did you check that your timing marks are actually at TDC on #1 cylinder?

 

[Russell]

yeah, that was the first thing I checked, cause I suspected a similar problem, when the timing mark is at 0, the piston has just hit TDC (as felt with a dentist's pick through the plug hole)

I ran my scanner on it, and it gave me a couple clues to look at...

When everything is hooked up, and it is sitting at an idle, the BLMs are approx 115 - 120. If I unhook the MAP sensor, that reading immediately jumps to 128, and stays there.

Tells me I have a vaccum leak, kinda interesting though, cause it has behaved this way since the start, but my BLMs never used to change on unhooking the MAP sensor...

 

[southernspeed]

I think your BLMs should stay at what ever they were before you disco'd the map but the Int's should lock at 128 as it prob puts you in open loop. Maybe the BLMs should too, not sure on that one.