swampyankee 91'

Ok so I have had my 91 w250 three speed auto ctd for about six months now and I f#@*ING love it! However when I bought it , the truck had a bad vibration while in drive or reverse and just idling(like sitting at a stop light). The guy I bought it from said it was bad motor mounts which I changed and the vibration is still there. I have read other threads on here of similar sounding problems and people have said it was a bad injector or a flex plate problem. I was wondering if anyone has had this same problem and what they did to fix it thanks!

dodgenstein

A lot of times, bad vibration while idling in gear is simply a matter of too low of an idle setting. Running A/C compounds the problem. I like to see about 800 RPM idling in gear w/ A/C on.

swampyankee 91'

Ok so if I was to turn up the idle it may help? I never run my a/c, because it does not work. And another stupid question, the idle adjustment is on the pump correct?

j_martin

It's on the pump. You need a tachometer to set it properly. Should be around 600 rpm or so.

You might also have a basic engine problem, leaking injector, worn injector, valve adjustment, or deeper mechanical problems like a bad camshaft.

What you have is engine misfire. It shouldn't do that.

swampyankee 91'

Ok thanks

swampyankee 91'

I have been doing some online research looking for a tach and have a couple of questions...what type of tach will work on my 91 first gen with 727 tranny, mechanical or electric? I have autometer ultralite boost and pyro gauges and would really like to match the tach to the other gauges if at all possible. Thanks again guys!

dodgenstein

An Autometer alternator tach is the choice of many on here. I believe the PN is 2388.If you can find someone with an optical tach, you can easily check your idle for testing purposes.

j_martin

The truck provides tachometer signal and power at a connector near the left end of the dash. Isspro and others make tachs that use that signal.

cougar

I wish my 91 had that provision.

dodgenstein

NonIC 91's don't have tach output connections. No CPS.

gyman98

What about your transmission mounts? Idle should be set to 750 in drive with ac on. As stated above you don't have a factory tach connector

willroach

Does the vibration go away once you accelerate in neutral above idle or once you pull away from the stop sign and start building speed? Mine did this and had same symptoms and it ended up being the IP and injector rework.

thrashingcows

I went with the Isspro Tach on my 89 motor. 2 magnets glued to your harmonic balancer 180* apart, then mount the pick up and run the wires. Dead nuts acurate and very simple.

j_martin

Forgot about that. Non-IC, or rather non-OD has no TPS. There's nothing electrical ticking with the engine rotation in the truck, so you'd have to stick on magnets or reflectors to run a tach pickup.

swampyankee 91'

@ gyman98, I did check my tranny mounts after I did the motor mounts and they looked pretty good...
@ willroach, the vibration does go away as soon as i touch the throttle at all, but I dont think its the ip, it was rebuilt right before I bought it this spring