95 Chev C1500 5.7L Runs Rough @ Low RPM High Load

I have a buddy with a 95 Chev C1500 5.7L 4L60e auto truck with about 150,000 miles. He's changed lots of parts on it. Plugs, wires, coil, EGR valve and solenoid, IAC. I only had a chance to look at it once for a short while. It drives pretty good and smooth. I loaded it up in drive with my foot on the brake and it revved pretty good. But if you sit in reverse with your foot on the brake and off the throttle then it runs pretty rough. The MAP will go to 12-13inHg, short term trim will be around 130 and long term will be around 140. Unfortunately his A/C system doesn't work but if you turn on every other load then the MAP goes closer to 13inHg and short term goes to 135 and long term went close to 160.

I'm not super up on these older chev TBI engines but I can't think of too many things that would cause those MAP readings. I checked the EGR and it's working totally fine. I checked the MAP sensor with a vacuum pump and it was solid. He told me that he tried cleaning the IAC before and that it was really dirty and seemed like it was sticking so I told him to throw a new one in. Wasn't very confident that'd be the problem but why not?

Afterwards he said he found a spark plug that he cracked while installing and it seemed to help a bit after changing it but still idled rough in reverse with foot on the brake. I'm still stuck on that MAP reading. Unless I'm misinformed I shouldn't think an ignition or fueling issue would cause a lower intake vacuum..? It'll idle rough but the RPMs won't drop too much.

The engine RPM is also pretty normal. If a vacuum leak of fresh air were causing that kinda MAP reading and the engine were compensating I would have thought the engine RPM would be higher and I'd have to hold the brake down harder to stay stationary. The thing should try and take off. Also I thought with vacuum leaks on MAP engines you had relatively normal fuel trims and just had a higher idle because the system is able to compensate for the extra air. UNLESS it does have some extra source of air entering and the computer is limiting the engine speed..? I'm not sure if these computers did that..? Plus it idles perfectly in park and neutral.

Can you have a bit of timing chain slack that effects idle under load yet doesn't make much noise? I didn't notice any ticking or chain noise. I wasn't particularly listening though.

Just wondering people's thoughts and ideas. I'm sure lots of you are way more familiar with these beasts than I am. I try not to touch anything not from this millennium! Attached are the engine performance wiring diagrams. Just ignore the manual transmission diagrams at the end. This truck is an auto.

Hi..

I think you are telling us the truck runs fine. If you put it in gear it does not!

If this is true I would unhook the break booster and plug the vac line and re test in gear. if the truck acts fine then you have a leak in the booster.

Doc n2mx

Haha wow! And that's why it can be good to ask outside people that haven't worked on the vehicle. To point out the obvious thing that you're totally overlooking! A leaking booster would make all kinds of sense. Especially if it only leaked when applied. Since he doesn't have a scanner I think I'll go over there and watch the fuel trims while clamping off the line.

Thanks for the suggestion. I'll let ya know what the outcome is.

Ugh, these TBI systems are useless. :sick: Soooooooo glad I wasn't around when these things were popular.

I see the fuel trims climbing... How does this truck drive down the road? Wondering if it's running out of fuel due to a weak pump/restricted injectors/ect. If it has plenty of power, then we can probably scratch that line of reasoning. If it falls on it's face, then maybe hook up a fuel pressure gauge?

You can also add alternative fuel to the intake and see if manifold vacuum improves. Surprisingly, a lean mixture and/or ignition timing WILL screw with vacuum levels.