1994 Chevy 2500 check engine light

[wallacesgarage]

hey guys i have a 1994 chevy 2500 6.5 turbo diesel Vin F 2wd automatic it has cruise control on it as well. Im having a weird problem with it when you hit 1700 rpms the check engine light comes on and you loose cruise control anything under 1700 the check engine light stays off and cruise control works any ideas on this would be greatly appreciated thank you

[DmaxMaverick]

Welcome aboard!

What's the code (it should be stored)?

The fuel filter and fuel lift pump operation would be the first things to check. If the fuel pump isn't pumping, it could be a failed pump, OPS (Oil Pressure Switch), fuse, or ignition switch. OPS failure is very common. You can check fuel pump operation easily: Route fuel filter water drain hose to a container (hose/valve is located at the thermostat housing), start and idle engine, open water drain valve. Fuel should flow into the container at a good rate, and the engine shouldn't stall or stumble. If there's little/no flow and the engine stalls, the pump is not pumping. If there's good flow and the engine stalls, the filter is plugged. The water drain is also a good place to check fuel pressure (if all other tests pass), just install a gage at the end of the hose and open the valve. It should show a steady 4+ PSI at idle, and remain above zero at all loads. The water drain is on the "dirty" side of the filter, with no components between it and the pump.

[wallacesgarage]

it shows no codes new fuel filters lift pump has been replaced, injection pump rebuilt new injectors trucks runs like a dream but when the light is on the fuel mileage goes from 18.5 to 14.5 does not affect the performance of the truck at all the light will stay on above 1700 rpms but below 1700 the light is off and doesnt store any codes at all

[DmaxMaverick]

Which light? The SES, or STS (Service Throttle Soon)?

A new lift pump doesn't mean it's working when it should. Verify with the fuel pump tests.

[wallacesgarage]

its the service engine soon light

[JohnC]

its the service engine soon light

Then there should be codes. How are you checking them?

Off hand, it sounds like you might have a wastegate issue, but you need the codes, or drive with a scanner to see what is triggering the light.

[phantom309]

If you drive with the foot feed, does it still trigger the ses ?? is it triggering the ses when it,s working hard to climb a hill? As was mentioned you may have an overboost situation if the wastegate is seized,. mine does it because i have a heavy spring on the gate, and and a bigger pump,.the engine defuels or cuts back, and i think it not only drops fuel off but limits timing, soon as i switch it off and restart the ses is gone,.power is 100%.

Nick

[Robyn]

Whatever it is, it's a transient issue and is not setting a hard code.

Are you using a scan tool to check things, or a simple code reader that flashes the dash light ???

If the mileage is goiing south this may be related to the waste gate/turbo and its controls.

When the light comes on do you see BLACK SMOKE out the tail pipe ??

If so, check the vacuum pump and related vacuum hoses that run from the pump to the waste gate and the little control solenoid on the LH valve cover.

Make sure that the vacuum pump is making about 20-26" HG at the pump.
If the vacuum reding at the pump is unsteady (vacuum gauge is bad wobbly) then the pump is bad.

Check the vacuum at the solenoid (LH valve cover near the back) should have full vacuum to the solenoid.

Check the vacuum at the waste gate acuator, should be steady and around 20" HG ++ with engine idling.

If this checks out OK so far, you need to get a manifold pressure gauige on the thing and find out whats going on.

Its also possible that the MAP sensor (Little rectangle shaped plastic box on the top of the intake air plenum) has gone whacky.

Make sure that with the engine off, that the turbo wastegate arm will move back and forth freely.

If the arm will move easily with the engine running and there is vacuum at the actuator, then the actuator is bad.

As far as the cruise control goes, the ECM will shut the cruise off when there is a CODE showing.

Keep us posted

Missy

[wallacesgarage]

i will check all of this out and let you guys know the vacuum pump is brand new just installed friday i will check it out and the waste gate and keep you posted on what happens thanks for your help

[DmaxMaverick]

The wastegate actuator should modulate to 15" vacuum. If it's at 20" (or full system vacuum), something else is wrong. If it's too high, however, this shouldn't cause any low-load issues (but can cause overboost conditions at/near max load). This is with OEM programming. If there's an aftermarket program (chip) in the works, the rules change.

[wallacesgarage]

truck is stock no chips