1988 Chevyenne driveability issue

[biggsteve97]

We have an 88 Chevyenne 2wd 350 TBI that weve spent the last two months tryng to get up and running. First the engine had no power, found the distributor was off 2 teeth fixed that and got the timing set to 2 degrees. Ran strong and very smooth until a couple weeks ago. If you are accelerating it runs fine, at idle its fine, in lower gear and rpms high its fine. But if you try to cruise and rpms are around 1000-1300 once operating temp is reached, the engine starts cutting out and running extremely rough. Above or below that it runs perfect. Checked timing, good. Checked for vacuum leaks, good. Cold then engine runs fine its just when it reaches operating temp and ONLY at 1000-1300rpms. Changed the TPS Monday night with no effect on the problem. Could this be a failing ignition module? Could by some weird chance since the fuel guage doesnt work, be low on fuel? It would be odd if it was since its fine at other rpms, I would think. Could it be a fuel pressure issue? I am going to work on it as much as possible on Saturday and even hook my laptop up to it to see what the computer is seeing/doing. Engine has unknown mileage, supposedly bottom end is rebuilt. Oil pressure is good. Did notice while engine is cold a little blue smoke coming from right hand exhaust, but goes away when engine is above 130 degrees. Any advice/thoughts is welcomed. Thank you.

[drumsdaddy]

Temperature sending unit?

[ChevyTech]

What you describe could be caused by a distributor that is so worn the shaft can wiggle sideways.

You could also try disconnecting and plugging the vacuum line to the EGR and taking it for a test drive.

[biggsteve97]

Originally Posted by ChevyTech

What you describe could be caused by a distributor that is so worn the shaft can wiggle sideways.

You could also try disconnecting and plugging the vacuum line to the EGR and taking it for a test drive.

Swapped temp sensor with a known good one, no change. Plugged of egr vacuum hose, no change. Checked egr valve and it was closed and relatively clean. Gonna put a timing light on it in the morning to see if timing is jumping around and make sure timing is still set right. I have wondered about the distributor with the truck having almost 230,000 miles and the last owner doing an engine swap cause he lost oil pressure and shot a rod. I am also going to connect my laptop upto it and see what the comp is seeing.

[biggsteve97]

Update: Last night got to tinkering with the truck. Found corrosion of 4 the terminals inside the distributor cap and the rotor showed signs of arching or burn out on its main tab. Also, tested the ICM and once it got hot it lost the reference signal on all subsequent tests. After replacing the cap, rotor, and ICM the engine starts quicker and runs smoother at all rpms until it starts to get warm. The roughness and chugging was still happening from about 1000-1300rpm. So I disconnected the egr vacuum hose and plugged it off just to make sure. So far, the roughness has stopped, so far. The egr appears to be closed and seated but it does return slowly to its seat position when I push it open and watch it close. Doesn't matter if the engine is running or not, it slowly returns to the closed position. Wondering if its closing to slow and that's causing the roughness. When the roughness happens, the exhaust smells extremely rich and the passenger side injector spray pattern is really strange. When the engine is running fine the spray pattern looks OK but doesn't match the driver side pattern. If the problem returns with the egr plugged off I will swap injectors out with another set I have and see what happens. I'm not sure what else to do at this point.

[Cabnchassis]

FYI that intake manifold can leak vacuum internally. If the thin little gasket between the air and coolant passages fails it'll pull extra air outta the coolant passage. Not the issue here but it's a good thing to know. I fixed all my vacuum leaks and still had a massive one. So at the advice of a TBI guru over at FSC i pulled the manifold and viola no gasket between one of the air/coolant passage at all.