Project: TBI Fuel Injection – Fixed

Back in the spring of this year I purchased a fuel injection kit for my Scout II from Affordable Fuel Injection (For those of you interested in the kit, it looks like it’s now only available through IH Only North).

Jason and I installed the kit this summer, but the truck never ran right. It always seemed to run too rich. Our first thought was a low vacuum signal due to the somewhat large cam I have in my 345. I contacted AFI support, and they told us it had to be a vacuum leak – the cam shouldn’t cause the problems we were seeing. We worked to find a vacuum leak, and finally determined the EGR valve was bad. We blocked off the EGR and improved the vacuum, but the GM TBI system still didn’t seem to work correctly.

The summer went on and we got busy replacing the floors. The fuel injection system took a back seat.

Finally, this weekend we decided to see if we could get it fixed. We installed the O2 sensor, which shouldn’t have been a major factor. The O2 sensor only works when the system is in the feedback loop. An engine will generally run without an O2 sensor, it just runs better when the sensor is working correctly. Again, the O2 sensor seemed to help, but things still weren’t correct.

The next step was to run through the AFI troubleshooting steps. One of the steps is to check the fuel pressure. The GM TBI system does not include a schrader valve to connect a fuel pressure guage to, and when we initially installed it an adapter to add an inline valve for testing. Since then Jason had acquired the needed adapter, and yesterday we were able to test the fuel pressure. After attaching the adapter to the pressure side of the TBI, we started the engine. Fuel pressure was measured at 40 psi. Correct pressure for the TBI system is 12 psi. Immediately we knew we were on to the problem.

Jason thought the TBI internal regulator should be control the pressure, and thought maybe the regulator was bad. The next step in the troubleshooting guide was to test the return side of the fuel system. We reconnected the adapter to the return line and started the engine again. The pressure? Again… 40 psi. Obviously this was a problem. We had some kind of blockage on the return side of the system.

Next step was to check the return line. I proposed we take the hose off between our new steel fuel line and the tank and run it into a gas can. This done, we started the engine – and found it ran MUCH better.

The troubleshooting guide continued with a recommendation to verify that the correct port was used on the fuel tank for the return line. A vent line may contain an orifice that would restrict return flow and cause a backpressure problem.

33 Gallon Scout II Fuel Tank
My Scout II has a 33 Gallon aftermarket fuel tank, like this one from This was a donation from my Dad’s project Scout II – Al. It was installed when we bought Al and he let me have it for mine. The tank is a great improvement over the 19 gallon stock tank, but is a bit of a nightmare to install. It’s designed to fit between the frame rails with only enough wiggle room to get the filler neck past the frame. At this point, we were sure we would have to pull the tank – not a fun job. We raised the Scout up and I examined the fuel line attached to the tank looking for a problem. Immediately I realized the problem was very simple. When we had installed the tank, the return hose had been kinked between the frame and the tank. I gave the hose a yank, pulled it out from where it was stuck, re-attached it to our steel line. Presto, problem solved.

My TBI fuel injection system now works as promised. For the first time since we built the 345 for my Scout actually seems to have a will to live. No more problematic carburetor, no more Gold Box ignition, hopefully no more engine related problems.

At this point, the only remaining fuel injection issues are mounting the ECM and checking the return line to make sure to flows to the bottom of the tank. Still have some floor work to do, and need to mount the seats, but then we can actually do some wheeling!

4 thoughts on “Project: TBI Fuel Injection – Fixed”

  1. Thanks for the info! I have been working on a 1972 chev pickup 4×4 resto. I Took out the “big block carburated motor”, and replaced it with a 5.7 TBI injected motor. I have been struggling
    with smoothing out the engine . It has always run rich and very erratic . I never got a check engine light,and everything checked out OK . I never thought to check out the fuel return line for resistance. I cleared the line & bingo ,shes 100%- thank you

  2. I had the same problem on my TBI system I have on my 1973 Blazer I fixed it by installing new return line tubing and hoses the old lines would randomly plug up so I figured it would be best to run new ones

  3. I have a 1988 5.7 tbi system. I have always worked with the older stuff. Can’t seem to get it to idle, replaced the idle air control valve, it stays running now but oscillates.

  4. Hi Guy’s
    94 GMC Yukon 383 roller motor forged 4L60E Trans… Trick Flow Twisted Wedge Heads $12 G’s Plus…in motor Motorvation convertion kit for GM TBI 2 barrel to a 4 TBI 750cfm…Four 75Lbs injectors… would not work… added a AEM Ser. II computer controller and coil on plug out of LS motor IAC will not work at all… and still running snotty
    100PSI pump intank and 4×4 low lock will not shift
    any HELP on this would be great…. Thank you in Adv. Ken 🙂

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