Engine starts and goes to full RPM???

[Dale A]

I have been working on my 91 R 3500 for almost a year with it up on blocks. Doing a transmission swap, and install all new tie rods, brake pads, rotors and so on. So just got the trans in, an '03 zf 6 and went to start the engine. It immediately went to full race. Shut it off just as fast, let it set, and tried again. Same thing. Disconnected the battery for 20-30 minutes. Again went to full race. I've checked vacuum lines, wiring connectors, test the sensors I could and or knew how to; MAP, IAC, TPS. I read stuff on YouTube and took that full circle with no new ideas to try or parts to swap out. My year long project didn't include any engine work or electrical other than extending the wires that connect to the newer transmission. One note here, can't do a engine running vacuum leak test, motor is at full RPM. Scared to let it run more than 30 to 45 sec. So I'm at my wits end. Anyone have any idea what's going on??? The motor is a 454 with TBI.

Also, to save questions and time I did do the code check and it came back just 12. Because I got a 12 can I assume the ECM is OK? I also disconnected each major sensor from the engine and started it with the same results each time, full race!

 

[Dmack]

Is it drive by wire, or does it still have a cable activated throttle? If cable type, is the throttle linkage stuck open?

 

[AuroraGirl]

I had this when doing my new manifold and carb,... might be the same...
Throttle return springs. I didnt actually get them in my kit for the truck, so I could barely get truck running and it would be at damn near redline. I Was inspecting to see if choke was doing something weird and noticed butterflies were vertical.

 

[Dale A]

Is it drive by wire, or does it still have a cable activated throttle? If cable type, is the throttle linkage stuck open?

I have been working on my 91 R 3500 for almost a year with it up on blocks. Doing a transmission swap, and install all new tie rods, brake pads, rotors and so on. So just got the trans in, an '03 zf 6 and went to start the engine. It immediately went to full race. Shut it off just as fast, let it set, and tried again. Same thing. Disconnected the battery for 20-30 minutes. Again went to full race. I've checked vacuum lines, wiring connectors, test the sensors I could and or knew how to; MAP, IAC, TPS. I read stuff on YouTube and took that full circle with no new ideas to try or parts to swap out. My year long project didn't include any engine work or electrical other than extending the wires that connect to the newer transmission. One note here, can't do a engine running vacuum leak test, motor is at full RPM. Scared to let it run more than 30 to 45 sec. So I'm at my wits end. Anyone have any idea what's going on??? The motor is a 454 with TBI.

Also, to save questions and time I did do the code check and it came back just 12. Because I got a 12 can I assume the ECM is OK? I also disconnected each major sensor from the engine and started it with the same results each time, full race!

 

[Dale A]

It is cable throttle. The throttle lever is pretty stiff, not sure how it could get sucked open, but I’ll check that out. It is otherwise closed in static position (with engine is off).

 

[AuroraGirl]

That tells us your engine is getting air somehow. Check for vacuum lines or open elements around the throttle body. If your throttle plate is closed, your idle air control is closed, and your vacuum lines are plugged or not a leak, there should be no way its getting to full RPM. I would suspect IAC being stuck open if there are no other ways its getting air sucked in. Id try having someoen start it and have air cleaner off, and quickly block the air cleaner openeing with something to prevent air flow. If it runs slowly or just period it tells you its getting air elsewhere. Im not sure what a R 3500 is, but if its a SBC im guessing its TBI and that really limits what it could be. Even if it was really rich it would stall before getting that high RPM. If you had an OBD1 capable tool(data streams were unreliable, uncommon on trucks let alone cars in 91) you could see if the IAC is being commanded open or its commanded closed and wont move or cant move.

 

[AuroraGirl]

An IAC should be fully "closed" when you pull it if this is the case. The pintle should be expanded away from the valve itself. Unless this doesnt use a pintle type. Im going off a 1990 3.8l v6 from an oldsmobile.

 

[Dale A]

I have been saying the same thing. How can it get to full rpm with the throttle plate closed. I will run those tests tomorrow, the blank off and mess around more with IAC. The “R” is the style of truck (3+3) or crew cab that was the last year made as a square body. They continued making the blazer, suburban, and crew cab as square bodies through ‘91. And yes it has the TBI and has the pintle type IAC and it’s on a BBC.
Thanks for staying with this and helping drill down on the problem.

 

[AuroraGirl]

NP and oh, didnt realize big block. I have no history or any work with them, but if its pintle I have loads. I had the problem you faced, slightly. It would eventually come down but it hanged for a long time. No amount of cleaning helped. Replacement helped. The issue was very rare after that. Turned out to be the ECU was defective. I tried the ecu out of a 1988 buick lesabre and transferred the PROM chip from mine. It fixed it instantly. did it in my HS parking lot.

 

[bucket]

Do you have a tach? A wide open IAC cannot flow nearly enough air to support high rpm. All it can do is just a very high idle. If the throttle blades are in fact closed and the engine is in fact winding out to speeds that could hurt itself, there must be two problems. Besides a very large vacuum leak, there must be an excessive fuel supply issue as well. Normally a vacuum leak big enough to support high rpm will just stall the engine out because there isn't enough un-metered fuel to support it.

So I guess my real question is, what kind of rpm are we talking here? If it's just 2k or so, I've often seen TBI engines cold start that high for whatever reason. It sounds like it's spinning really fast because the cooling fan makes so much noise. Then it settles down after a little bit of run time.

 

[Dale A]

Found the problem and analyzed why I couldn’t spot the problem. Nevertheless I’m still quite embarrassed to admit this to the world but if it helps the next person then I’ll take the lumps accordingly.
When you are restomoding a different tranny that requires a new cross that is home made or modified pay attention to the attitude of the engine. It only has two mounts and those mounts without a tranny connect let the engine rock forward and backward. I didn’t quite have my engine tipped back (bell housing end down) far enough when I measured for the rebuild of the tranny cross member. So when I put everything together and was ready to start the motor the fan blade had to do a little shaving on the lower portion of the shroud. What an awful racket. It sounds uh, kind like a run away motor.
Thank you mr. bucket for the tach idea. The motor was just at high idle, about 1350 rpm’s. Knowing that I (we, I had some man help today) just let the motor run and shortly there after the fan clutch released, came a stop and it got real quiet except for the purring of a sewing machine. Oh wait that’s the engine running at a normal idle.
All the sensors seam to be working ok. Actually identified nine to ten different sensors on the motor through this ordeal. The was kinda good I guess. Thanks to AuroraGirl for her help and insite.
So ya I feel like a dork but sometimes you’re just to close and can’t see the whole picture. ‍

 

[80BrownK10]

Do you have a tach? A wide open IAC cannot flow nearly enough air to support high rpm. All it can do is just a very high idle. If the throttle blades are in fact closed and the engine is in fact winding out to speeds that could hurt itself, there must be two problems. Besides a very large vacuum leak, there must be an excessive fuel supply issue as well. Normally a vacuum leak big enough to support high rpm will just stall the engine out because there isn't enough un-metered fuel to support it.

So I guess my real question is, what kind of rpm are we talking here? If it's just 2k or so, I've often seen TBI engines cold start that high for whatever reason. It sounds like it's spinning really fast because the cooling fan makes so much noise. Then it settles down after a little bit of run time.

I was going to say the same thing. No way Idle Air will suck that much. After all it helps control idle speed not total engine speed.

 

[Driver]

I experienced the same problem....bad ECM