1988 5.7 to 1995 7.4 swap question

[ARCTICboy]

Hello. New guy here. Have done a search but could not find a specific answer to a question.
I picked up a 1988 R2500 3/4 suburban 4x4 and somewhere along the line the 5.7 In it was swapped with a 1995 7.4L.

It’s running super rough and was wondering in doing a swap like this would any of the wiring harness, TBI computer need to be updated for a 7.4?

I have so far changed plugs, wires, distributor, cap, rotor, and check fuel pressure. It’s still starts ok but idles surging and the odd time stalls when put into drive. Under throttle it bogs but will maintain a highway speed after a while with only the slight amount of surge.

Before I got to far into parts and repair I thought I would check to see if the swap was done correctly. What should I look for?

Thanks for reading this long post.
Marcus.

 

[Vbb199]

1. Sounds like a vacuum leak to be honest

But If it isn't, the only really significantly special things that sets a tbi 454 apart from a tbi 350 are the injectors, the tbi itself, and its ECM (smog/egr) stuff included but rarely is that an issue

I'm sure connectors may be differently placed, etc etc

Is the IAC valve connected like it should be?

Is the timing bypass switch unplugged, or plugged in?
Its over by the brake booster

Is there any SES lights?

There's one immediate way I can tell you if it's a 454 tbi or not.

Send picture of the tbi itself, maybe on the throttle linkage side.

 

[ARCTICboy]

Hi Vince.
Thanks for the quick reply.
No SES lights at all. I will double check the IAC valve and the timing bypass switch and send a pic.

I ordered a IAC valve using the VIN and they sent me one that was for a 5.7, that was the tip off that the engine was switched. As well as reading up there were no 7.4's in the R2500 4x4 and didn't even think that the TBI might still be for a 5.7.

 

[R Carnella]

If it's a 4x4 Wouldn't your truck be a V2500?

 

[Bextreme04]

If it's a 4x4 Wouldn't your truck be a V2500?

This is correct.

7.4L(454) was only available on R20 Suburbans(2wd)
See Page 9: https://www.gmheritagecenter.com/do...ion-kits/Suburban/1988-Chevrolet-Suburban.pdf

 

[Bextreme04]

Also, an R2500 would be a 2wd GMC Suburban. At some point in the 90's, they all became 1500/2500, but for almost all squarebodies, the 10/20/30 will be for chevrolet and 15/25/35 will be for early GMC with 1500/2500/3500 being later 80's GMC

 

[Vbb199]

Hi Vince.
Thanks for the quick reply.
No SES lights at all. I will double check the IAC valve and the timing bypass switch and send a pic.

I ordered a IAC valve using the VIN and they sent me one that was for a 5.7, that was the tip off that the engine was switched. As well as reading up there were no 7.4's in the R2500 4x4 and didn't even think that the TBI might still be for a 5.7.

I believe a IAC and tps for a 5.7 are different from a 7.4L .

As I recall, a 5.7 tbi can fit on a big block, but a big block tbi cannot fit onto a sbc intake without boring the 2 ports out a little.

As I said, I believe the IAC and tps are different for sbc and bbc throttle bodies.

It's sounding like someone put the 5.7 tbi on top.

Some pics of it could confirm this

 

[Rusty Nail]

Its all about the pix, noob.

Haha! Yeap. My thought involved different fuel pumps between small and big blocks.. Not enuff volume though pressure may be fine I believe that reading may be misleading.

BUT!
Vince may also be on to something but I cannot confirm , that sure sounds legit though.

 

[Bextreme04]

Its all about the pix, noob.

Haha! Yeap. My thought involved different fuel pumps between small and big blocks.. Not enuff volume though pressure may be fine I believe that reading may be misleading.

BUT!
Vince may also be on to something but I cannot confirm , that sure sounds legit though.

Same fuel pumps, different regulators. The 1995 454 TBI was 30PSI. 454 ONLY, the 350 still used the same 9-13 psi from 87-95.

If that is actually a 1995 454, I really hope that the computer and harness were also swapped over.... because if not, it is gonna run like crap if it runs at all

 

[Vbb199]

... because if not, it is gonna run like crap if it runs at all

Well that's why the thread has been created lol

It sounds just like the PO put a stock 350, 490 cfm tbi on top of a 454.

 

[Bextreme04]

Well that's why the thread has been created lol

It sounds just like the PO put a stock 350, 490 cfm tbi on top of a 454.

Maybe.. although if they did that it would probably run OK. I would bet the PO did way worse than that and just put the entire 454 in there without changing the ECU. Since the throttle bodies and fuel pressure are totally different, it is running like hot garbage.

 

[SquareRoot]

O

Well that's why the thread has been created lol

It sounds just like the PO put a stock 350, 490 cfm tbi on top of a 454.

oohhh, the plot thickens. Imagine if someone swapped vortec heads on it. @Rusty Nail would have a serious knicker twist. lol

 

[Vbb199]

Maybe.. although if they did that it would probably run OK. I would bet the PO did way worse than that and just put the entire 454 in there without changing the ECU. Since the throttle bodies and fuel pressure are totally different, it is running like hot garbage.

This now makes 4 people patiently waiting for photos OP

 

[Vbb199]

@Catbox needs to post that pic with the pirate "this thread is worthless without pictures!" LOL

Not really, OP.

But some photos would help diagnose this.

 

[Catbox]

Your will be done.

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