Flat Spot when Accelerating

[60Works]

'88 FJ-62 with 300k on the drive train.

It's a tired truck that wasn't given enough love over the years. I've been slowly working her over and have finally gotten her save to drive. After sitting for a year with only monthly starts I've started driving it again on test runs, trying to get her up to DD status for a short commute.

Problem: there's a flat spot on the torque curve between 1,200 and 2,400 rpm. She runs great and pulls very strong above 2,400 revving freely to 4,000 and has reasonable pull below 1,200. With the pedal to the floor from a stop, she will accelerate slowly to 1,200 then just sit there. It happens in every gear at precisely the same range.

When I slowly let off on the pedal and it reaches the range where a slight acceleration would normally occur, it starts to pull 'correctly'. Any more pedal and she flops. This makes me think mixture but is it fuel or air and what controls it.

The EFI is a new area for me. Could anyone please start me in the right direction?

Thanks

 

[cruisermatt]

Could be throttle position sensor. You've probably done this but just make sure all your vacuum lines are going the right direction, check and make sure none of the wire harness is burnt through, and after looking inside my 275K throttle body you should probably clean that and your upper intake manifold out, they will have a huge carbon build-up inside them.

If none of these are the issue at the least they can't hurt.

 

[jcj_78FJ40]

That's happen to me a couple of times. 1st time it was a clogged diverter valve to the EGR. There's an easy test for this in the manual. The 2nd was a bad cat clogging the exhaust.

 

[vipergrhd]

Bad accelerator pump in your carburetor man,needs a complete rebuild and bad knutsen valve in your variable cam sprocket and last not the least
your muffler bearing is falling apart bro.




























Just joking bud.

 

[Wallygator]

You probably don't even have an issue. First of all acceleration and the 60 series is a oxymoron. My truck is in pretty good condition and every part of the torque curve is flat.

 

[LAMBCRUSHER]

fj62...tps check and adjust. make certain there are zero intake leaks from air cleaner housing on. make certain the egr isn't stuck a tiny bit open. jump the e1 and te1 terminals in the diag box and count codes. baseline tune-up time? IE, plugs cables rotor, valve adjustment, air filter...fuel filter, fuel pressure regulator and its VSV...if I could get it, so can you. these are super easy to transition into efi with...they'll lead you to hate carbs...my 250K+ used replacement 3FE just passed cali smog yesterday

 

[cruisermatt]

You're in New Hampshire right? Just de-smog it, it makes working on stuff so much easier. I also spent this morning sand blasting my upper intake manifold (air intake chamber in the FSM) which had literally 1/8l to 3/16" of carbon buildup inside. I wish I could do that on a high mileage rig and see what difference that alone makes.

 

[60Works]

This is a old thread but, in case someone finds it in search when trying to solve a similar problem, I thought I'd share the solution.

Changing the fuel regulator and fuel pulsation dampener solved the hesitation issue. I changed them at the same time so don't know which one is the true solution.

 

[micruz60]

This is a old thread but, in case someone finds it in search when trying to solve a similar problem, I thought I'd share the solution.

Changing the fuel regulator and fuel pulsation dampener solved the hesitation issue. I changed them at the same time so don't know which one is the true solution.

Makes sense to replace the two items together, given the age and deterioration over many miles.
Thanks for sharing your solution!