New rebuild rpm surge when blower is turned on

[Wilcox]

I just put a new rebuilt 2.9l in my 1988 ford ranger 4x4 xlt extended cab. Fired off for my 20 min break in and at 2,200 rpm runs smooth but when I turn blower fan get some heat in the cab the RPM started jumping all over the place. Turn fan off and back to smooth idea. Did this a number of times and same result? New battery New altinator New distributor all new tune up. Has anyone had this problem?

 

[RobbieD]

Can't say that I've had, or heard of, this exact problem, but the very first thing that I would check is that ALL of the grounding cables from the battery negative post are in place, and in good condition. One large cable should go to the frame and then on to the engine block; a smaller cable should go to the inner fender or the radiator support; and especially check that a grounding strap is present from the firewall to the engine. That last is commonly forgotten after engine work or engine swaps, and it not being there can cause a lot of screwy symptoms. The blower motor is a large, "noisy" draw, so I can see it not having a good circuit path* causing an issue.

* Always look at electrical as having a "circuit path" from battery positive to the component and then back to battery negative; the grounding side is as just important as the powering side.

 

[CamTheHedgehog]

Can't say that I've had, or heard of, this exact problem, but the very first thing that I would check is that ALL of the grounding cables from the battery negative post are in place, and in good condition. One large cable should go to the frame and then on to the engine block; a smaller cable should go to the inner fender or the radiator support; and especially check that a grounding strap is present from the firewall to the engine. That last is commonly forgotten after engine work or engine swaps, and it not being there can cause a lot of screwy symptoms. The blower motor is a large, "noisy" draw, so I can see it not having a good circuit path* causing an issue.

* Always look at electrical as having a "circuit path" from battery positive to the component and then back to battery negative; the grounding side is as just important as the powering side.

+1 on the block-to-firewall ground and its screwy symptoms. That connection was loose about a year ago on my ranger and caused an occasional no start, as well as the speedometer fluttering from 5-20mph while I was parked at a complete stop. Ended up taking it to the local shop to have them fix it because it drove me up a wall. (That was before I took my advanced electrical class at college)

 

[SenorNoob]

I second that opinion...

 

[Wilcox]

I double cked all grounds and are all good. Just took for test run and as soon as you give it Gas the rpms start jumping around for a second then smooths out give more gas and dose it again even a little back fire. Timing is at 12 ° could mass air low cause this? I also unhooked blower motor and it did make any difference. Very weird.