I recently bought a 1989 dodge D100 from a guy in West Virginia. He was the second owner and his father-in-law was the first. The body is super straight, the upholstery is good except for a small hole in the drivers seat. The truck has 154000 miles. I pulled it home to Georgia with a tow bar. No problem. I plan on bringing it back to some of it's former glory with a little body work and paint, new upholstery and tires and rims. I started with getting the AC going. It needed an evaporator and now it blows an artic breeze. Here is my problem I need help with. Although it seems to run smooth, it won't get over 65 unless you're going down hill. So far I've checked the timing and set it at 10 degrees with the temp sending unit unplugged. I checked the temp sending unit's resistances and they are in spec. The compression on all cylinders is 135-145. The fuel pressure is 14 psi. I checked for a plugged CAT and the back pressure is about 0.5 psi. I looked at the injector spray and it didn't look bad, no drips, for what that's worth. For good measure I have ordered a MAP and a TPS. They should be here next week. I've read everything I can on the net and I can't come up where anyone else has faced this issue. I read in one technical book that the SMEC {computer} on this truck has two programs, Run and Start. The Start sets the fuel and timing etc while the engine gets going. After that the Run program takes over and controls the engine functions while operating. This article also said that if the Run program crapped out that the Start program would take over so you could drive it to the shop but with poor performance and fuel economy. Could the truck be running in this Start program and that's why it is running like this? I don't want to go just changing parts until I find the right one so if someone has some wisdom on this issue I would really appreciate it.

 

if your just going to throw parts at it i would recommend replacing the o2 sensor and i would even go as far as recommend moving it to the y-pipe instead of the manifold. i feel like the manifold may be too close to the engine to have the o2 sensor work properly. there is also a extension cord on the o2 sensor that goes from the rear passenger side of the engine to the drivers side o2 sensor. it almost seems like they were planning to put them on the y-pipe but didn't for some reason. these things also seem to preffer the ntk brand o2 sensors.

 

It already has the O2 sensor in the Y pipe.

 

Don't be so sure there isn't an exhaust problem. I got a reputation back in the 80's when I had a shop and we found a problem the dealers couldn't solve. I got a 16 year old Buick that ran great around town. Get on the open road and try to pass and it would fall on it's face. It had been to 2 dealers and a couple of regular shops. We ended up taking the hood off and mounted a large vacuum gauge on a bracket on the engine so we could see it on the road. Vacuum was 17 inches idling or down the back roads. When we got out on the expressway, it still ran good. Kicked it down to passing gear and it began to crank, then fell on its face. All the way back to the shop it bogged and sputtered. Up on the rack, I had one of the guys floor it until it bogged. I drilled a 1 inch hole in the down pipe just below the manifolds. I got an explosion of pressure and the engine smoothed out. The owner showed up as we were putting the hood back on. I called a buddy with an exhaust shop down the road. We didn't do much exhaust work, and definitely not the type this car would deserve.

His total cost was $175 for a new exhaust and $75 for our shop time. His down pipe from the engine was double walled and a flap of metal had rusted loose. Under acceleration, it would flip up. When it cooled, the contracting metal would pop down. He had spent nearly $1000 in 1980 dollars trying to fix a car he had bought new. I got swamped with work after that.

My point (and I do have one, I just usually wear a hat so you can't see it) is check the basics. It may be something simple. It could be a restricted exhaust, dirty fuel filter impeding flow but not pressure, or something like that. I had a problem recently that was a broken hold down clamp on the distributor. Something I've never seen on a street engine before.

 

Good point on checking the simple stuff. A couple things I did that I didn’t mention. I took the O2 sensor out and went down the road but got the same performance. Also I read the vacuum as I reved the rpm up to a couple thousand and the vacuum did what it was supposed to do. But I’m going to take your advice and check the vacuum from inside the cab at 65 mph and see what gives. Also checking the fuel pressure at speed is a good thing to try. Maybe it is breaking down at higher speeds.

 

You have a separate fuel filter that is mounted on the frame rail. Replace it and you should be good. I had a 90 that started the same issue around the same mileage. You may have good fuel pressure but under load you don't have enough volume threw the filter.

 

The fuel filter was changed not many miles ago by the previous owner but stuff happens so I hooked up a pressure gauge with a long hose and ran it into the cap and observed while going down the road. The pressure never changed even a little bit from idle to 65 mph.

 

I checked the vacuum from inside the cab with a long hose and when I hit the gas anywhere above 45 the vacuum would go to nothing until I let off the gas then the vacuum would go to 23 inches until I reached a steady speed and depending on the speed the vacuum would level off at 12 to 15 inches. What do you think?

 

Probably a partially plugged exhaust. Either your catalyst has broken down in the converter or the muffler is either rusted inside and collapsed or it's full of rodent bedding. With the exhaust cool, crawl under and thump it (<------------ high class diagnostic procedure) with your fist. You may hear a rattle. Rust is the biggest possibility if the truck sat a lot or was only driven short distances and never cooked the moisture out of the exhaust system.

 

Another thing you can do is look down the throttle body and rev the motor. You should be able to see two good streams of fuel coming out of both injectors.