350 TBI performance...

Cheekster96 · December 13, 2004

...or lack of.

hello everyone, one more newbie, hope you all can stand it.

I have a 87 350 TBI in a 3/4 ton (HD 9600 GVW) 4x4 truck with 4.10 gears and the TH400 tranny.

Recently, i installed a new Edelbrock performer intake, Hedmen Headers (long tube) and new valvetrain (new single springs, pushrods, locks and seals). While the heads were off, i spent the weekend doing a little porting and polishing job of all the ports and combustiion chamber. I have experience in this. I had installed a new Holley TBI unit but one of the o-rings burst and spewed gas all over the engine compartment (my air cleaner assembly was soaked and a nice flame appeared.). I covered the spark plug wires in header wrap to protect from the headers.

The truck fires right up and runs ok.

Here is what it is doing and what i did afterwards...

I set the timing (with a light) on the notch that says 5 before, on the plate above the balancer, (to believe it is 10 degrees.). the truck seemed to idle best here too and drove the best here as well, so i left it alone.

I can see gas being dumped from the injectors into the throttle body (still, stock assembly is on) and have recently changed the oil. My oil pressure sucks (15psi @55mph and barely above 0 at a stoplight in gear, so i put it in neutral). those figures are for a warmed up engine, so i know fuel is being wasted and getting into the engine. I havent had a problem with compression in the past and the rings were fine.

going from a stoplight and maybe 1/2 throttle, the truck will stumble and sometimes want to stall out.

I have another throttle body from a 87 305 GMC i am goin to use and see if that makes a difference.

The truck is throwing no codes and i put my little code reader on there anyhow and showed nothing.

So, again, getting bad performance and even worse mileage. i used to get 12 mpg and now when i filled up, i got 7.5!!!

I didnt install the Edelbrock cam and matching lifters, will do that when i overhaul the engine.

I am about ready to give the truck to the local mechanic and see what he can do with it.

Also, i have disconnected the AIR system piping. The EGR is still functioning fine, the MAP sensor is good as is the TPS and IAC.

If i can get the Holley TBI fixed, imight try that again.

Any suggestions?

Sorry for such a long post being my first post. i just want to try and solve this problem before dishing out alot of money to someone else.

Thanks all.

Lee

I am having some terrible driving conditions now. It seemed before that the truck made alot more power.

jake_33 · December 14, 2004

I have an 89 it also has thotle body injection 350 i to can c gas spewing into my carb but other wise my truck runs fine, but i did however encounter the same kind of problem as u when i first got the truck it had factory exaust on it and i was getting about 13 mpg then after putting single 3 inch straight pipe on it my mpg has droped to about 11 and for oil presure mine had the same prob until i changed out the senser but u said yours isnt breaking the codes so i dont know jus my 0.02

citydriver · December 16, 2004

Almost sounds like the engine is starving for air...after you ported/polished the heads and installed the new exhaust the stock TBI ubit may not have enough CFM to let the engine breath....I would also set the BASE TIMING back to 0*...you will have to disconnect the EST connector to do this, usually a single wire with a small connector that comes out of the wiring harness (sorry I'm not sure of the location on that year truck) As for the oil pressure, have you checked it with a MECHANICAL GAUGE? The dash gauge may not be accurate...just a thought

Cheekster96 · December 16, 2004

thanks for the inputs. i ll take all that i can get.

the oil pressure gage worked fine prior to any of the mods i did. I bought some higher viscosity oil and will see if that makes a difference too. but i havent used a mechanical gage. where would this hook up to? the wires were not pinched when i installed the new intake manifold, was careful and took my time about all of that.

i have looked for this wire that people have talked about, the shop manual says it is a tan wire with a black stripe along the side of it and is connected to the firewal goin to the distributor via a wiring harness. i looked, there is no such wire or harness or any wires goin to the distributor. i have the distributor that has the separate coil mounted. there is one wire that has a connection (like a test port or something) that is not connected, but is a pink color if i remember correctly.

when i had the timing light on it before and had adjusted it to 0*, the engine would kick and stumble and eventually die. i tried to restart it and it would back fire and a pretty orange flame shoot up thru the throttle body, i didnt want to try that again.

also, i did some research about modifying the TBI engines, since i didnt do a camshaft install, i dont think i need to modify the ECM that is in the truck??? Of course, getting it done wouldnt hurt, but as with everyone, money is tight right now. im just trying to get the truck to run like it was, my goal was not to decrease performance. wanted to be able to tow my boat up the mtns alot easier, now, i can barely climb (unloaded) the hills here at 45 where i was able to do it before at 55 (unloaded).

i think you may be correct Citydriver about the air issue. I have a Holley TBI (aftermarket 750 cfm i think) which doesnt work. one of the o rings busted and shoots fuel from the passenger side injector and they wont cover it because i bought it over a year ago (and cant find the receipt, my own fault). so, i m gonna try and repair that too.

any more idea? like i said, i ll take all the (good..lol) advice i can get..

thanks all.

Lee

citydriver · December 21, 2004

You should be able to find the EST by-pass(tan wire w/white stripe) coming out of the main ECM wire harness that runs through the passenger side of the fire wall....it should by taped or cable tied outside of the harness split-tube..the color of the wire may be hard to notice due to the age of the truck....it may also be INSIDE of the cab under the glove box BEFORE the harness plugs into the ECM.....Trying to set the base timing without disconnecting this connector would affect your perfomance and may be the main cause of your performance problem.....as far as the Holley TBI...you should be able to get a seal kit for it through Summit Racing or even a Holley dealer in your area......Best of LUCK to ya'

superbee · December 21, 2004

My '88 acted really strange until I realized my voltage was very low (11v). I put a new ground strap from engine to frame and frame to battery and it was MUCH better. This caused my Oil pressure fluctuations to disappear also. If you need a place to attach your mechanical gauge, just remove the oil pressure sender and use the fittings included with most gauges. Just a hint, the plastic tubing is ok for a temporary check, but I always use copper tube for permanent installs, it's much tougher. Good Luck.

Cheekster96 · December 24, 2004

Thanks gang. i will try all that advice. Havent been out on the truck much at all, due to the holidays and work. I will look at it on sunday most likely and let you all know how it goes..

Thanks again and Happy Holidays.

Lee