Another SBC TBI Problem Thread (1 Viewer)

[PadreCruiser]

First of all, let me apologize because I am not a mechanic and I have a lot of information to convey so I hope it makes sense.

Problem:
My car is a 1968 FJ40 with a Chevy 350 TBI swap. It starts and runs but it overheats and once it is warm (not overheated) it backfires through the intake under load. The engine doesn't feel particularly powerful at any time, but especially once it warms up. Also, once it is warm it will not start again if you turn it off, the starter runs and runs and the engine never starts (there is spark, confirmed with timing light).

Backstory:
I bought my 1968 FJ40 in 2011. It came with a 1971 SBC 350, Carter 600cfm AFB carb, Offenhauser intake, HEI conversion, and SM465 to stock 3-speed transfer case. After I rebuilt the carb it ran great on the roads, except I live less than an hour from the Fordyce Creek area and love the mountains. The elevation and angles in rock crawling made it impossible to keep the carb.
So, I did a junkyard TBI conversion with parts from several different folks on Craigslist, but kept my original heads and block (I will call this Conversion 1). In this conversion it just never ran right (overheating, backfiring through intake) so I bought a complete motor/harness/computer out of a 1995 K2500 truck thinking that at least I would know that all the parts are from the same vintage. I ended up swapping the top end and wiring from the 95 onto my original block because the old block was in better shape than the 1995 block (Conversion 2).
I put it all together and it ran first try, but immediately began to show the same symptoms.

This leads me to think that there is some condition that exists in both my first and second TBI swap.
I have checked the fuel pressure and rebuilt my TBI pressure regulator. Fuel pressure is solid at 14-15psi during idle and revving. I have also tried three different fuel pumps and the problems do not change. The fuel return flows freely back to the tank through a "T" at the overfill/breather hose.
Vacuum is steady at 18 in/Hg.
Just today I confirmed that timing is set at 0* with the wire unplugged and that when piston #1 is at TDC the rotor is pointing at the wire for #1 in the distributor.
I am stuck. I have checked the CTS and the readings are correct. I have also replaced the TPS and checked a lot of other sensors that I can't remember right now.

There is a lot more information, but I hope it is a good place to start. I am somewhat at the end of my rope, I have invested a lot of time into this cruiser and I just want to wheel. Thanks in advance for any help.

 

[reddingcruiser]

First thoughts:
Cam shaft is not correct for the TBI set-up.

Are you running O2 sensor(s)?
What other sensors are not hooked up?

 

[whitey45]

O2 sensor cannot be too far downstream, should be right at the collector of the manifold? What intake? Using an adapter? Cam would have to be pretty aggressive to cause a problem.

 

[PadreCruiser]

Thank you guys for chiming in so quickly.
First off, I am using the O2 sensor that came off the donor truck. I will measure how far it is down the pipe, but it is not far below where the ramshorn manifold comes into the downpipe. The cam, intake, and heads are from the 1995 K2500 and are stock.
For sensors I am using every sensor on the intake manifold (CTS, TPS, MAP, IAC, etc.), also I have the Knock Sensor and VSS hooked up. I am getting a ton of codes related to transmissions because I am still using my SM465 and the K2500 was an auto (4L60 or something like that).
I would love for it to be the O2 sensor because that did not change in location between Conversion 1 and 2. Off to measure where it is.

 

[PadreCruiser]

The O2 sensor is 3inches below the drivers side exhaust manifold.

 

[reddingcruiser]

This was on my clipboard, it had some good info when I had my TBI, particularly with regards to cams and cam timing.


350 TBI Modifications That DO NOT Affect The Computer - K5BlazersPlus

350 TBI Modifications That DO NOT Affect The Computer - K5BlazersPlus

 

[jcj_78FJ40]

Are you sure you have the correct injectors? The 4.3, 5.0 & 5.7 injectors are the same size but have different fuel rates.

Also, did you swap out the HEI dist for a EST dist? The EST modules are known to be problematic.

 

[whitey45]

That is ok, map sensor can cause a lot of issues, but you probably have the one with the 95 conversion.

 

[TwelveGravity]

Best of luck to you. Wish I could be more helpful. I replaced about every external part on my 95 TBI as I chased everything around.
TPS, IAC Valve, Knock sensors, Rewired the knock sensor as they can fault anywhere between, MAP, replaced the distributor and many other things that I am sure Im forgetting. The computer I have is from Howell EFI and is a stand alone unit since I wasnt running the 4L60E. They were very helpful to walk me through things.

 

[RAYJON]

I always go back to the basics, spark , compression and then fuel. Start with the ignition system ohn wires check cap and rotor , plugs...etc..test any sensors then compressio/ leakdown test...last fuel,

Backfire= , timing, plug too hot, mix too lean, spark crossfiring, vaccum leaks, burnt valve,

Once you figure it out get someone to reprogram the eprom, for a manual vehicle and clean up the tune, you will like it alot more then...good luck

 

[PadreCruiser]

Thank you all so much for your questions and suggestions.
Jcj_78FJ40--I will double check my injectors, one came out of my Conversion 1 and one came out of Conversion 2 (there was a broken post on one of the Conversion 2 injectors). Both motors were supposed to be 350's. I will check the numbers on them. To another question about injectors, they both seem to have good conical spray patterns when viewed with a timing light and neither drip.

When I switched to TBI I did use the TBI distributor, and I have actually tried a few distributors to make sure mine was not faulty. Same results with each.

Whitey45--I will test out the MAP sensor today and let you know the results.
ReddingCruiser--thanks for the info on TBI Mods.
TwelveGravity--I feel your pain. It is like running through a rabbits warren at times with how many directions the problems can go. What ultimately solved your problems?

Thanks again.

 

[PadreCruiser]

I always go back to the basics, spark , compression and then fuel. Start with the ignition system ohn wires check cap and rotor , plugs...etc..test any sensors then compressio/ leakdown test...last fuel,

Backfire= , timing, plug too hot, mix too lean, spark crossfiring, vaccum leaks, burnt valve,

Once you figure it out get someone to reprogram the eprom, for a manual vehicle and clean up the tune, you will like it alot more then...good luck

Thank you for the suggestions RAYJON. I replaced the plugs wires, and distributor a few months ago, but I will ohm test them to confirm. I did a compression test they were all within 10% of 130psi, I did not do a leakdown. I just readjusted my valved too and all seemed to be moving just fine.
I am buying a kit from Moates to reprogram my eprom as soon as I scrape together the pennies.
I have checked for spark crossfire and do not observe any when running the engine at night. Also I have tested for vacuum leaks, but not found any yet.
Thanks so much, it is motivating me to get working on this again, I was about to give up.

 

[74fj40]

I am buying a kit from Moates to reprogram my eprom as soon as I scrape together the pennies.

Before you spend the money on a moates kit, you may want to look into EBL Flash from Dynamic EFi. It's a new computer which allows tuning on the fly, and many more features that a stock ecm.


EBL_Flash

 

[RAYJON]

There is also a guy here and one over the hill that used to do custom chips for them for 100-200$, been a couple of years, well worth it to pay for it.

search on pirate4x4, the one up here is a jeep guy he could even program the 305 units to work on a mild 350..

also be aware the factory roller cam 350's have a different cam gear, its softer for the roller cam it will wear more with the wromg gear..

 

[RAYJON]

Ohh i forgot, check the baseplate on the throttlebody, the gaskets can go bad,

The only problems i had with mine in 8 years was the fuel pump 3 times, baseplate gasket/isolator and 3 coil wires.

Everytime the gas mileage dropped the coil wire was bad..

Good luck, might be better to hit pick and pull on the half price day, tomorrow and grab a whole unit. And a eprom, maybe you will get lucky and there is a manual truck there...

Sounds like the unit you have a hodge podge,

 

[PadreCruiser]

Before you spend the money on a moates kit, you may want to look into EBL Flash from Dynamic EFi. It's a new computer which allows tuning on the fly, and many more features that a stock ecm.


EBL_Flash

Thank you for the info 74FJ40. That looks like a cool setup.

 

[PadreCruiser]

There is also a guy here and one over the hill that used to do custom chips for them for 100-200$, been a couple of years, well worth it to pay for it.

search on pirate4x4, the one up here is a jeep guy he could even program the 305 units to work on a mild 350..

also be aware the factory roller cam 350's have a different cam gear, its softer for the roller cam it will wear more with the wromg gear..

Thanks Rayjon. When I had the motor on a stand and mostly disassembled I saw that the cam was a standard hydraulic one, I also have it mated to the heads that came with it (1995). I did replace the cam gear and timing chain (double roller) for the appropriate year.
I will look up those chip burner guys. Thanks for the suggestion.

 

[PadreCruiser]

Ohh i forgot, check the baseplate on the throttlebody, the gaskets can go bad,

The only problems i had with mine in 8 years was the fuel pump 3 times, baseplate gasket/isolator and 3 coil wires.

Everytime the gas mileage dropped the coil wire was bad..

Good luck, might be better to hit pick and pull on the half price day, tomorrow and grab a whole unit. And a eprom, maybe you will get lucky and there is a manual truck there...

Sounds like the unit you have a hodge podge,

It is somewhat of a hogepodge, but most nearly everything from the top end up is from the 1995 K2500 350. The only part that is not, or is not new is one injector from my earlier swap.
I might just swap out the baseplate gasket though, it is one of the few things I have not done.

 

[PadreCruiser]

That is ok, map sensor can cause a lot of issues, but you probably have the one with the 95 conversion.

I checked the MAP sensor and it tested fine. I wish that was the issue.

 

[PadreCruiser]

Are you sure you have the correct injectors? The 4.3, 5.0 & 5.7 injectors are the same size but have different fuel rates.

Also, did you swap out the HEI dist for a EST dist? The EST modules are known to be problematic.

I did swap out distributors for the EST unit, and have tried 3 different EST distributors on this motor with no change in problems. I will check on my injectors. Thanks.