TBI 400 Small Block

**Xenon** · 02-25-2012

Ok, a little backstory and then questions that will no doubt demonstrate my ignorance.

Got this 'burb in '09. Had a burnt up 5.7l (worst seized engine I ever saw, ran with no water in the radiator)

I got a 400 to put in and decided it'd be neat to go EFI. Put all the tbi components on the 400 exactly as they came off the 350, not quite knowing what I was doing. It ran rather well, had a dead spot off idle and never quite felt like it was putting out as much power as it could until above 3000 RPM. got 10-12 MPG highway, which I thought was stellar for a 7500 lb truck with 4.10 gears. I attributed the less than perfect performance to the fact that it had the stock 400 cam designed for a carbureted engine, something I had overlooked when I put it together.

Somewhere around Feb. 2010 the truck flat died going down the road. I immediately suspected the ECM, as I had been experiencing rough idle, severe surging (45mph to 65mph and back without moving the accelerator) and excessive fuel consumption (5mpg)

The ECM1 fuse was blown. There was no fuel coming from the injectors. It had spark though, it would kinda cough to life if you sprayed alot of ether in it.

Replaced the fuse. Nothing. It did not blow again though, which ruled out a shorted wire in my mind. (EDIT: In my thinking, it ruled out a shorted wire. It in no way ruled out a shorted wire in my mind...)

I was very short on money at the time, I was in Arizona and trying to save up to come to ND for work... So I took a weekend and put a Holley on it with the carb'd manifold that had come on it.

It's never been the same without TBI. doesn't have the power it used to, gets 7mpg uphill, downhill, headwind or tailwind.

I have the time (and some money) to put into it now, as well as a '98 Suburban to drive daily, making this somewhat of a project truck now.

I am here, of course because I want to put TBI back on it, but I want to do it right.

I have the original TBI components off of it, and the TBI components off of a near identical '88 K20 Suburban that I scrapped out for a guy a few months ago.

I have 2 1227747 ECMs, one that I believe to be bad (the '87) and one that I assume to be good, but don't actually know.

the '87's ECM has an AFCB prom and the '88's ECM has a ADYR prom.

My basic idea was to put a 1987 5.7l cam in and put the ECM from the '88 in and plug it all back in.

Please point out all the flaws in this idea and offer your better ideas.

**93V8S10** · 02-25-2012

Sounds like you've get a plan, just need to get it all back together, so you can work on diagnosing the original problem. Don't worry about the stock 400 cam, it will be just fine, as evident by how well it ran with stock tune.

**EagleMark** · 02-25-2012

Originally Posted by 93V8S10

Sounds like you've get a plan, just need to get it all back together, so you can work on diagnosing the original problem. Don't worry about the stock 400 cam, it will be just fine, as evident by how well it ran with stock tune.

Agreed!

Your origanal issue, since you had spark but ran with ether was a fuel problem. So could be faulty relay, plugged fuel filter faulty ful pump?

You ran good on a stock 5.7L chip on a 400 engine is probably why you ran out of power at higher RPM.

Rough idle, surging and poor fuel milage could all be tied together to a vacuum leak.

If you put it all back together we can diagnose what went wrong, fix it, then move you on to tuning the chip for a differant motor.

I don't see any flaws in your plan. Just have to pay attention to detail like vacuum leaks when you do the intake change and TBI install. Rest is pretty simple plug and play.

**Xenon** · 02-25-2012

Great!!! I was thinking I'd have to go with the stock TBI cam... this, then, is not necesary?

What do you think about the ECM1 fuse blowing? I'm pretty excited about getting this this TBI'd again. Part of the reason I stopped driving it and got the '98 was the poor fuel economy with the carburetor... Also, it's a 4x4 and the '98 isn't, and being in ND the 4x4 is much needed in the winter. Except for this winter...

I'm almost done with the bodywork... fixed the rust and am going to paint it when the weather turns good.

Also just completing an axle swap from the 4.10s to 3.73s...

**gregs78cam** · 02-25-2012

When you get ready to put a 4L80E in, I have some ideas and tips for you, and if i can find the website, a guy talks about how he adapted the TH400->NP205 adapter plate to work on the 4L80E. But I concur.....get that beast back together, and let go from there.

**Xenon** · 02-25-2012

Great... It's all about MPGs for me right now, as the 400sbc is more than enough power for daily driving and occasional trailer towing without much extra help... So the 4L80E swap is high on my list once I get it figured out and gather all the parts...

**EagleMark** · 02-25-2012

Your cam ran good before so it's a good EFI cam. The tune for more cubic inch will help. I wouldn't change a cam for your reasoning...

Suburbans have plenty of room for bigger tires! Stay skinny and go taller and you'll have same overall ratio change instead of changing gears. Tires will be cheaper and WAY easier!

**Xenon** · 02-25-2012

Ok. I was just worried that the dead spot off idle and the fact that it didn't really open up till 3000 rpm was because of the cam...

Though I've read alot on SBCs since then and they do need to rev more to make power compared to a big block...

Mainly the big dead spot was my worry.

Another thing it used to do was under some conditions (uphill with a load, say) you could floor it and it just didn't have the guts. Then it would shift down and rev up, and then it would really rocket away. Thinking back, this was likely because it was not sending enough fuel for the 400...

Thoughts?

**EagleMark** · 02-26-2012

Big dead spot could have been a mechanical issue showing up that ended up failing? Could have been the tune was for differant engine? If the engine is sound and mechanical issues are in order it can be tuned!

If it's still running do a vacuum test. What is the reading? Is it steady? Also can do a compression check. You can't tune out a low cylinder compression reading or a bad valve...

**Xenon** · 02-26-2012

It was a machine shop reman when I got it and I'm confident it's in good shape. ~20,000 miles on it...

Question: It has a Holley aluminum intake on it right now with a spread bore pattern... would it be worth it to get an adapter plate for the TBI unit and add the coolant temp sensor or should I just change the manifold back? I wouldn't think twice about it except that it has the 400 heads which have the old style intake bolt pattern. It was quite an ordeal to get the tbi manifold to bolt down properly. It'd be a pain to reproduce what I did the first time...

It runs really well as is with a Holley 4bbl carb and HEI distributor. smooth and powerful. but it's lacking compared to the TBI and seems like such a stone ax... also the TBI always started in the cold (-15F) where as the Holley is a bit of work to get going in the frigid temps... The 7mpg with the Holley is what really kills me, and I've tried all sorts of things with it to improve. Holleys aren't known for being the most economical anyway...

Thanks for all the answers so far!

**EagleMark** · 02-26-2012

I would use an adapter! Just make sure you install without vacuum leaks and the TBI bolts don't go through adapter and hit intake pushing adapter up casuing a vacuum leak!

Coolanr sensor needs to be in waterflow when thermastat is closed, near thermastat is best place!

**Xenon** · 02-26-2012

If memory serves me, the Coolant temp sensor is the only thing I would loose by not switching manifolds... Is there anything else on/in the TBI manifold that wouldn't be on the carb'd one?

**EagleMark** · 02-26-2012

EGR? Exhaust cross over from heat riser valve on one side of exhaust? Don't know what intake you have? Both those help MPG and cold warm up and drivability...