Rebuilding TBI on 454.

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Big D

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Im gonna rebuild the TBI on my truck, theres slight hesitation in some areas of the powerband, where power tends to drop off, I figure nothing can be lost from rebuilding a TBI that could possibly have 148k miles on it and never being rebuilt.

Is there a way to tell if its been rebuilt? I know on some old Q-jets there was a couple things you could look at to see if someone cracked it open before.

damn rebuild kit is way more expensive then I anticipated, 90 bucks on rock auto.

CFM technologies offers the adjustable fuel pressure regulator, has anyone had any experience with these?

I checked out the TBI mod thread. other then smooth around the area where air enters, not much applies.

Should I repalace the IAC valve? do they go bad? clean it at least?

Thanks
 

dave_erald

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Typical things that fail are the pressure regulator spring and diaphragm, even if they look good toss them.

Before blowing it apart hook your timing light up and point it at the injector spray pattern, give you a fair idea on whether the injectors can reused or tossed.

The throttle blade shaft is bad for wearing out and causing a vacuum leak out the ends. When running spray some carb cleaner at the out side of the body directly at the shaft ends, provided the throttle body gasket to intake is good and the seals are bad you will notice it act like a typical vacuum leak. I believe the only way to fix that is hog out and hack in some brass bushings or replace the throttle body entirely. Holley still sells a replacement I believe.

I couldn't guess what the 454 body and injector numbers are but they are both definitely bigger than small block stuff.

As far as whether its been apart before, how nasty is it? Usually the pcv vent is plugged solid. Like black, thick and solid mate, you're gonna have to dig that **** out.

What else am I missing?
 

Big D

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thanks for the info. what a mess, the throttle blade shaft wears out!!! I saw some kits online that uses brass throttle bushings.

It visibly doesent look all that bad, but it doesent look like it was rebuilt recently either, if ever.

almost seems like I might be better off goin Fi tech, then dealing with this Poorly designed TBI.
 

MIHELA

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If you replace it, don't throw the old one out. They are still worth a decent amount and hard to find.
 

Big D

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well did research on the Fi Tech, that wont be a walk in the park in the "Land OF No" more or less extremely difficult, so thats not gonna happen.

Ill spray it today and see if it leaks, my truck idles a little high around 885 RPM and has a pretty rough idle in drive occasionally. also one of the machined holes right above the driver side throttle blade makes a high pitch whistling sound.
 

Justin S

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I recently rebuilt the one on my 350, the kit for that one was under $30, so there must be a difference between the SBC and BBC. I only did it because I had it apart and the o rings around the injectors were all stretched and I couldn't get them back together, so if you do start to pull it apart, there's probably no going back. I also did the "Quarter Mod" on mine while apart, i've heard that some people have had noticeable gains from it, but I really didn't notice anything, mainly because I was sorting out other issues at the same time.
 

Big D

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I like the adjustable pressure regulator from CFM Technologies and the inline fuel pressure guage adapter setup...

https://0190c50.netsolstores.com/cart.aspx

as far as the Rebuild kit, Rock auto and amazon want upto $120 for the rebuild kit, but ive seen it for 40-50$ on ebay.... I dont understand why theres such a huge difference in price, its a bunch of gaskets and o-rings and what appears to be a fuel pressure Diaphragm and 2 screws on ebays picture and 2 needles on rock auto's picture.

http://www.rockauto.com/en/moreinfo.php?pk=984599&cc=1034424&jsn=541

http://www.ebay.com/itm/REBUILD-KIT...ash=item33a82baec5:g:7GgAAOSwTapV3gT-&vxp=mtr

http://www.sprperformance.com/#!services-1/jgesc
 
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Big D

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Typical things that fail are the pressure regulator spring and diaphragm, even if they look good toss them.

Before blowing it apart hook your timing light up and point it at the injector spray pattern, give you a fair idea on whether the injectors can reused or tossed.

The throttle blade shaft is bad for wearing out and causing a vacuum leak out the ends. When running spray some carb cleaner at the out side of the body directly at the shaft ends, provided the throttle body gasket to intake is good and the seals are bad you will notice it act like a typical vacuum leak. I believe the only way to fix that is hog out and hack in some brass bushings or replace the throttle body entirely. Holley still sells a replacement I believe.

I couldn't guess what the 454 body and injector numbers are but they are both definitely bigger than small block stuff.

As far as whether its been apart before, how nasty is it? Usually the pcv vent is plugged solid. Like black, thick and solid mate, you're gonna have to dig that **** out.

What else am I missing?

Ive been checking this listing out, seems to handle all the issues you described. im gonna call later today to see how much He'll buy mine off me for. see if I can put a dent in the price.

what do you think?

http://www.ebay.com/itm/50mm-2-7-4L...ods-/262123218690?vxp=mtr&hash=item3d07c2e702
 

DerekTheGreat

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I've got a 454 with similar problems to yours. Idle is 800 RPM from the factory. From what I was told, anything lower will wear out the camshaft. Don't believe what the tach in the cluster tells you. Mine was rebuilt by SpecMo and reads ~200rpm higher at idle, so it says I'm idling at 1000rpm when it is actually 800. I used an old school sweeper style tach and then a MT2500 scanner when I got it to see what it was. Up until that I went crazy swapping out different PROM chips and such and even tried playing with the base idle, which was already messed with by the PO. Every time I'd plug the IAC in after closing it by grounding out the A&B terminals the RPM would slowly find it's way back to what is actually 800rpm (didn't know that until I hooked the scanner up to it)

Anyway, I did a partial rebuild on my TB with new gaskets, fuel pressure regulator & full clean out. Throttle shaft was fine. Didn't fix the "flat spot" with the gas pedal and still hesitates while accelerating. So, I swapped out the IAC (no difference & the RPM still "hangs up" when shifting gears- with the IAC closed & unplugged it does not) Checked my TPS, voltage was within tolerance and wasn't erratic when going to full throttle. Checked my lines to the MAP sensor & swapped that out. Things got a little better - found someone had tee'd into another manifold vacuum source- bad. It should go from the middle of the back on the TB directly to the sensor. Then I replaced the 02 sensor, as my truck has lots of power and runs fine up until it starts to warm up. Got a little better but still goofy. Replaced the fuel pump because it **** the bed. Much better but still there. Full tune up plus A/C Delco reman'd distributor & set the timing to 6*. No difference. Now I'm thinking that perhaps something is going on with the EGR, that perhaps it isn't closing all the way so going to investigate that. If nothing pans out there then I'll assume I've got a loss of compression due to a buggered up valve or something. Not sure what gives and I've asked around.
 

dave_erald

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Ive been checking this listing out, seems to handle all the issues you described. im gonna call later today to see how much He'll buy mine off me for. see if I can put a dent in the price.

what do you think?

http://www.ebay.com/itm/50mm-2-7-4L...ods-/262123218690?vxp=mtr&hash=item3d07c2e702

That's a hard one to sell for me just for the simple fact that you have already priced out something (FI TECH), that for another 300 dollars will outperform that newer TBI. And there is also the factory Vortec system that when converted to use a 0411 Pcm would walk all over that TBI unit.

Probably the best answer I could give you is decide what you are going to do with that 454 mill first, and if any of the future modifications include heads or camshaft swaps then don't even waste any money on the TBI injection, it won't keep up. However, if you are going to leave it stock, or very near stock, and if this guy is going to give you money for your TBI core then I would say yes, giver and do the ebay replacement. That ebay replacement looks like he has everything covered as far as replacing and upgrading is concerned and when priced separately without the new throttle bushings is probably the same if not more.

***A fair warning though, its pretty safe to assume that upping the injector size and doing mods on a throttle boy injected big block is going to make it that much more difficult to pass a smog test in the future, if at all. I imagine there is a balancing point somewhere, though I haven't the foggiest where it would be, but I would be hesitant going BIG on TBI injection and then asking it to pass smog***

The cheapest answer by far is if you find your TBI isn't worn out, and isn't leaking anywhere odd, then I would throw in a new regulator, possibly new injectors (nevermind, a flow matched set for sale on ebay is 211 dollars.... yuck) and drive it.

Based on your smog numbers it's a fair assumption to make that you are already starting with a healthy big block. And that's a good thing.


If you have the time and you know of some people (or others can chime in hear anytime) I would ask some others how there trucks run with bigger Throttle Bodies and bigger injectors, and if any of them have passed smog (I am willing to throw money down that they haven't and can't, and until they show you smog tests proving otherwise I would assume no)

I would save the money and do a Multi Point Fuel injection swap, or step up to a FI TECH or Atomic EFI, but that's because if I had a Big block I would be throwing heads, cam and intake at it...but that's me.



I've got a 454 with similar problems to yours. Idle is 800 RPM from the factory. From what I was told, anything lower will wear out the camshaft. Don't believe what the tach in the cluster tells you. Mine was rebuilt by SpecMo and reads ~200rpm higher at idle, so it says I'm idling at 1000rpm when it is actually 800. I used an old school sweeper style tach and then a MT2500 scanner when I got it to see what it was. Up until that I went crazy swapping out different PROM chips and such and even tried playing with the base idle, which was already messed with by the PO. Every time I'd plug the IAC in after closing it by grounding out the A&B terminals the RPM would slowly find it's way back to what is actually 800rpm (didn't know that until I hooked the scanner up to it)

Anyway, I did a partial rebuild on my TB with new gaskets, fuel pressure regulator & full clean out. Throttle shaft was fine. Didn't fix the "flat spot" with the gas pedal and still hesitates while accelerating. So, I swapped out the IAC (no difference & the RPM still "hangs up" when shifting gears- with the IAC closed & unplugged it does not) Checked my TPS, voltage was within tolerance and wasn't erratic when going to full throttle. Checked my lines to the MAP sensor & swapped that out. Things got a little better - found someone had tee'd into another manifold vacuum source- bad. It should go from the middle of the back on the TB directly to the sensor. Then I replaced the 02 sensor, as my truck has lots of power and runs fine up until it starts to warm up. Got a little better but still goofy. Replaced the fuel pump because it **** the bed. Much better but still there. Full tune up plus A/C Delco reman'd distributor & set the timing to 6*. No difference. Now I'm thinking that perhaps something is going on with the EGR, that perhaps it isn't closing all the way so going to investigate that. If nothing pans out there then I'll assume I've got a loss of compression due to a buggered up valve or something. Not sure what gives and I've asked around.

DerekTheGreat, and soon to be greater, I would definitely do a compression check (including a leak down check), and a fuel pressure check (even though you did just do a fuel pump).

When you do go to order a EGR, it should be fairly simple for them to get you the right one, but I do remember having to dicker with parts guy on getting the correct EGR for GM vehicles before, as some are positive back pressure and some are negative back pressure. I don't what the hell was going on but just remember this being a problem, so if you swap it out and it gets waaaaay worse that might be why.

Here's a good explanation on the different types

http://www.aa1car.com/library/egr.htm

Most TBI engines are notable for plugging the EGR routing in the manifold itself. That's a quick check by grabbing the diaphragm inside the EGR while running and pull up, Does it die?

The shifting gears and rpm hang up crap, do you have a TH400? If so is the vacuum line plumbed correctly and it is not sucking trans fluid through a ****** modulator?


There are probably three or four different threads that could be started from all of this...I need a coffee
 
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