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Gary, ASE Certified Technician
Category: Chevy
Satisfied Customers: 917
Experience:  Senior Technician
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I have a 1991, C2500, 454. Engine was operating fine,

Customer Question

I have a 1991, C2500, 454. Engine was operating fine, @55mph, coasted downhill and applied throttle on uphill...engine bogged to barely running. No codes. Replaced barely out of spec MAP sensor. All other sensors check ok with volt/ohm meter. New plugs, wires, and rotor. Coil pak ok. Fuel pump @ 31 psi. Engine runs fine on CALPAK,(MAP disconnected). New ECM with old EPROM installed...same problem. Help!! Thanks.
Submitted: 9 months ago.
Category: Chevy
Expert:  Gary replied 9 months ago.

Hi David,

Can you check your fuel pressure on the road?

Sounds like a dying fuel pump.

A gauge taped to your windshield will show if the pressure is low on the road.


Customer: replied 9 months ago.
I have not checked the fuel pressure while traveling but with MAP sensor disconnected I'm able to give full throttle and the engine responds well. No bogging, hesitation, or attempted backfire.
Expert:  Gary replied 9 months ago.

Which is disconnected on the map sensor?

The vacuum source or the electrical connector?

Customer: replied 9 months ago.
The electrical source is disconnected. A new piece of hose is connected to the vacuum. I have also rebuilt the pressure regulator of the TBI (large spring installed).
Expert:  Gary replied 9 months ago.

That is interesting.

Do you have a scan tool that can monitor engine data?

You have answered my next question about the hose, you must be a mindreader.


Customer: replied 9 months ago.
I do not have a scan tool for this computer...I have to read codes with a paperclip. I'm concerned that the eprom I moved, to the new ECM, is faulty, but I don't like being a parts changer. Almost everything done so far could be considered an extensive tune-up to a 25 year old vehicle but I would prefer to put my finger on the problem. Thanks.
Expert:  Gary replied 9 months ago.

Can you borrow a scan tool?

Always better to look at engine data.

You are really guessing without seeing what the ECM sees.


Customer: replied 9 months ago.
I'll locate one. ( Hope it doesn't require a floppy drive :) I will be in touch with new information. Thank You
Expert:  Gary replied 9 months ago.

There is lot of scan tools around now.

A GM scan tool would be best, ***** ***** of a lot of generic tools that can show multiple ecm data would be good.


Expert:  Gary replied 9 months ago.

Hello david,

Do you need any further help?

I hope my advice has been of assistance to you.

Kind regards