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Rough Idle and Running Rich, '90 4.0

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I just replaced the 4.0 in my '90 Pioneer with another one. Engine was running fine (except for the rod knocking) running on all 6, smooth idle, plenty of pep. Well, the swap went pretty smooth. Only mod I did to the engine, put in new 4 hole injectors. Had to pull them out, two injectors started spraying gas everywhere (had cracks in the bodies). Put the old injectors back in, no leaks. Get ready to fire it up, now it idles really rough (sounds like a healthy race cam in it) exhaust is extremely rich (old injectors are single hole). I used the same throttle body that I took off the old engine. New vacuum lines/harnesses, plugs, wires, cap/rotor, fuel filter, air filter, battery cables.

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I replaced all the vacuum lines (new harnesses and a few of the lines that didn't come with them) and this is one that I replaced with all new line and fittings. I have some pics I took of the engine before I pulled it. Will compare them with the way it is now. Also have schematics and diagrams I printed out to work with. Wish me luck, and Cruiser's Tips is the "Bible" for me :)

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Happy dance here, got Ol' Blue's troubles fixed. Double checked everything, vacuum lines, fittings, connectors, grounds etc.. MAP sensor vacuum line was broken at the throttle body. Thankful that I kept all the old vacuum lines and harnesses, had to make one from scratch-----and it worked like a champ. Idles smooth and runs awesome :) Here's a pic of the line I made and one of the engine before I dropped it in Ol' Blue ('90 Pioneer, 4.0, auto, tilt, cruise).



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Glad you found it. Ever see this?



The Renix throttle bodies have a strange and failure prone connector on the side where the MAP supply originates and then runs up to the MAP sensor located on the firewall. This hose/pipe assembly is no longer available for purchase.

The real kicker here is how critical this line is in supplying the correct vacuum signal to the MAP, the most relied upon sensor for the ECU to read regarding air/fuel ratio. Any cracks, melted spots, or loose rubber connectors can cause major starting and driveability issues.

There’s a simple fix though. All that’s required is a 1/8” NPT tap, a new throttle body gasket ( Napa FPG 60742 ), a vacuum fitting (Napa 05703-B102), two vacuum elbows (Napa2-670), and a length of new plastic piping (Napa 2-672).


Remove the throttle body and take it to the workbench.

Using an oiled tap along with a driver, carefully thread the lower of the 2 holes of the throttle body where the old fitting was plugged in. Don’t go too deep. These are pipe threads.

Flush the hole with carb cleaner and inspect for any left over cuttings.
This is an excellent time to do a complete throttle body and IAC cleaning.  See Tip 11.
Take the vacuum fitting (05703-B102 ) and apply a LITTLE bit of thread sealer on the threads only. I prefer Permatex #2 but almost anything is fine.

.1-TB to MAP hose-001

Carefully screw the fitting in until snug.

Install one of the vacuum elbows on the MAP sensor so it points toward the throttle body, and the other vacuum elbow on your new throttle body fitting so it points up to the MAP sensor.
Cut a length of the new plastic tubing (approximately 13 inches) to fit between the vacuum elbows and install it making sure there is enough slack for some engine movement. Route it according to the photo. We don’t want any rubbing or chafing with engine movement.  Not a bad idea to use some contact cement or Gasga-Cinch sparingly on the tubing to elbow connectors.

1-TB to MAP hose

Revised 2-06-2016

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