No power at highway speeds

[Doubletop]

At my wits end. 1987 1500 305 TBI engine. A few months ago, the ICM went bad. I screwed up and broke the connector inside the distributor so I ended up replacing the whole distributor. The problem the truck is having now is that it runs great speeds in town, but I've lost all power at highway speeds and fuel mileage has dropped from 12mpg down to 8.

So far to try and find the problem the following items have been replaced.
- Distributor (Changed it twice since problem appeared after initial change. Bought top of the line from Summit)
- Rebuilt throttle body
- A/C Delco new fuel injectors
- Fuel filter
- O2 sensor
- Knock sensor
- MAP
- TPS
- EGR Valve
- EGR Solenoid
- Plugs, wires, and coil
- Catalytic converter and muffler
- Coolant temp sensor

EGR valve and solenoid are now testing good. Vacuum reading at idle is a solid 19 with no movement at the needle during idle. I've also sprayed cleaner around throttle body and vacuum connections and not finding any leaks. Timing set at 0 with wire unplugged. Good fuel pressure before fuel filter. Compression is good, but have not down leak down test. I've cleaned and redone the ground wires at thermostat housing. No codes are being thrown. It's not missing out. Fuel injectors good cone spray pattern. I don't have smell anymore so I can't tell if I have a fuel odor at exhaust.

Couple things I noticed. Cruise control stopped holding speed at the same time. It still engages, but will slowly lose speed. I took apart the cruise solenoid and rebuilt that according to a video I found online. I also noticed today that when I had blower motor on vent (no A/C) the fan would slow down and nearly stop at higher rpms. I know that's normal on some newer vehicles to help with power, but wasn't sure if that was normal for this year of truck.

Any ideas would be greatly appreciated cause I hate taking my stuff to be worked on by someone else.

 

[gmbellew]

sounds like a vacuum leak at the reservoir or the cruise control vacuum hoses or the cruise control solenoid itself or somewhere in the plumbing of vacuum hoses. that vacuum leak will also cause you to have less power at highway speed when engine vacuum is overall lower already and the vacuum leak makes it even lower.

 

[Ricko1966]

So the truck ran great until the ICM died? This is a new problem after changing the distributor?Is the fan motor actually slowing down or are the blend doors closing.

 

[gmbellew]

So the truck ran great until the ICM died? This is a new problem after changing the distributor?Is the fan motor actually slowing down or are the blend doors closing.

if it ran great before ICM and then had this problem after, i bet a vacuum hose came off or got torn when he was messing with the distributor...a lot of vacuum hoses for cruise, ac, booster, etc all on that area.

was the ac fan stopping or was the air being redirected to the floor vents?

 

[Doubletop]

Yes truck ran great till then. And really I guess I didn’t check if blend doors were closing or slowing down. Just noticed air flow reduced from vents. I can check that tomorrow

 

[Ricko1966]

if it ran great before ICM and then had this problem after, i bet a vacuum hose came off or got torn when he was messing with the distributor...a lot of vacuum hoses for cruise, ac, booster, etc all on that area.

was the ac fan stopping or was the air being redirected to the floor vents?

Yep,that's what I was thinking. That's why I asked about the blend door. Yea just saw you were typing your response same time as me. Looks like we are both on the same page.

 

[Doubletop]

Is that normal on these to close blend doors at higher rpm’s? Or is it because I’m losing vacuum and it can’t hold the blend door open?

 

[Ricko1966]

The blend doors operate off of vacuum. So does the cruise control. And with a vacuum leak your map sensor won't read engine load correctly so fuel and spark will be incorrect,in closed loop.

 

[Doubletop]

Ideas on locating possible leak? At using port on throttle body the vacuum gauge doesn’t move so if I have a leak it’s only showing at higher rpm’s. I’ve checked all hoses in engine bay. I’ll bypass the cruise servo to see if it’s internal of that but any other ideas? Besides modulator on tranny are there vacuum lines anywhere else at the transmission?

 

[gmbellew]

brake booster and the vacuum feed that splits off into the AC, cruise, reservoir are the main ones. other possible locations are EGR, MAP, fuel cannister. manipulate the vacuum hoses after you have turned off the engine. the reservoir should hold some vacuum and you will hear it releasing.

 

[Doubletop]

Well, I found a vacuum leak at the cruise control release switch at the brake pedal and it appears the vacuum hose at the manifold may be collapsing. I replaced the hose at the manifold, but have to wait for new switch to come in. Thanks for the help

 

[Doubletop]

Well, it's doing better, but not 1oo% yet. Got a smoke tester and the only smoke I'm seeing in the vacuum system is coming from the throttle body most likely from the IAC area. Is this normal or should the IAC pintle be sealing this area off? I replaced all the vacuum line under the hood with fuel hose cause I was finding the smaller diameter vacuum hose was collapsing when the engine bay was warm. My idle is great and still drives great in town. Just still having problem with full throttle power and gas mileage.

 

[YakkoWarner]

The IAC pintle is normally slightly opened when the key is turned off, the only time it is supposed to be 100% closed is when they key is on and the ALDL jumper is put across the pins to blink out the stored codes.

It may be worth jumpering the ALDL, letting the codes blink out and while they are flashing disconnect the IAC's wiring plug. Then you can turn the key off and smoke-check the vacuum system. That is also how you close the IAC when setting base idle.