91 4.0 hitting rev limiter too early | Ford Explorer - Ford Ranger Forums - Serious Explorations

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91 4.0 hitting rev limiter too early

59Bisquik

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I have a 91 4.0 in my B2. It has no codes, passed smog and runs well. However, in 1st and 2nd gear at WOT, it will hit the rev limiter at a little over 4500rpms. I have to back off the throttle just a bit and then it will shift. The tranny does have a shift kit, but I cant imagine the rev limiter is set at 4500. Any ideas?
 


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The rev limiter kicks in just below 5000 RPMs. It's supposed to so the engine doesn't fly apart.

There's no point in revving the 4.0L OHV beyond it's max torque of 2400-2500 RPM, especially in an automatic.

Best bet for performance is to use a KKM intake and cat-back exhaust to free up the bottlenecks so it will get up to max torque faster. Revving it up like it's a NASCAR V8 or some high-revving import just wears the parts and wastes gas.

If it's not shifting correctly, even when driven normally, it's a transmission issue rather than a rev limiter issue. Low fluid level is a possible culprit and easily checked and fixed.
 




59Bisquik

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This issue only occurs when at WOT. Under normal driving, it is not an issue. However passing on the freeway, merging onto the freeway etc, commonly requires WOT with my big heavy B2.

I wasnt sure what the rev limiter was actually set at for the 4.0. The tranny was originally built for the 2.9 and the shift kit allowed it to rev a little higher than normal. So without dismantling the tranny and pulling the shift kit out, is there anything else I can do?

BTW, it has a free flowing exhaust but still has a stock intake. A bigger MAF and a CAI are coming soon. Anything for a little more power and better mileage...
 




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Did you swap in the gauges from the 91, or are you sure the reading on the tach is correct?

You might also check the throttle cable itself, and the throttle body. If the cable isn't tensioned correctly, even with the pedal fully depressed, "WOT" isn't available since the throttle body isn't ever truly wide open. Hence the "zip tie mod" on Explorers for a stretched gas pedal cable.

The rev limiter on the 4.0 OHV is set just below redline, which shows as ~5400 on some tachs, but is usually just shy of 5000 on the 91-94. The engine will rev freely up to the limiter in neutral, but hitting it in gear is still pointless for performance in terms of acceleration. Letting the transmission shift up around ~3000 RPM and going for quicker shifts from 1600-3000 RPM is a better way to get up to speed rather than some monster rev job until it hits the limiter every time. I know some people drive like this, but it just makes noise.


Check the fluid level in the tranny. Check out the fluid and be sure it's not black/brown or burnt. You might also want to be sure the engine is in top shape, with maintenance items like cleaning the MAF sensor, throttle body, and making sure the spark plugs are in top shape.


Bigger MAF's don't always help. Without internal work or better flowing heads (stock heads on 91-92's are weak and prone to cracks anyway), and a cam to go with it, a larger MAF can just screw with the ECU and actually give worse overall performance.
Aftermarket heads, a cam (maybe), and headers will go a long way to improving overall mid-top end performance if that's where you want the torque and power. The stock 4.0 has it's torque down very low.

A CAI like a KKM, or even a cob-jobbed adapted K&N cone filter will make a big difference. Sounds badass when revving, too.
 




59Bisquik

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I did not swap the gauges, due the roll cage it was easier to keep the original dash. However, when it is idle, it is correct and verified against the smog machines tach. And at freeway cruise and it is correct according to the gear calculator. However, up in the 5k range...unknown.

The zip tie mod was done back when I had the 2.9...not sure what that would change since the computer doesnt control the shifts. But a thing to look at.

Tranny fliuid is clean and full...matter of fact, always run a quart or quart and a half overful for verticle climbs. New plugs, wires and clean MAF, just took it in for smog...no codes either

As for the power increases... I had read that the bigger MAF could possibly help with MPG. Im not trying to make it fast...I realize its just big heavy and slow. Just better than 10mpg around town would be nice. I have gotten 15 with straight freeway driving and a relatively flat freeway. My old 2.9 got 9mpg all the time.

Just wondering how to fix this issue...its really bugging me when hitting the limiter before it shifts. I dont want to remove the shift kit because I really like the firmer shifts and less tranny heat due to slipage. Not to mention, in the dunes, having to back out to shift is a real PITA. Wonder if a chip with a higher limiter is the easy solution.
 




jd4242

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ive had this problem for months now and no one can figure it out.many just point to the astg tranny manual and tell me to trouble shoot from there but it hasn't helped.i believe mine is a vac problem.do you have an adjustable vac shift module?do it do it with the kick down cable off?here is the other threads.http://www.explorerforum.com/forums/showthread.php?t=278153
 




59Bisquik

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JD4242...
I now know that the tranny is the issue. The tranny isnt malfunctioning, its doing what its supposed to do with a shift kit installed. But what is the best way to fix it? I was concerned that it was something wrong with my swap but it looks like this engine just doesnt rev as high as the 2.9.

Would adding a chip with a higher rev limiter fix the issue and who makes a good chip or tuner. As stated previously, its big heavy and slow...Im ok with that, just want to fix the rev limiter issue and any MPG gain is a huge plus!
 




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My first guess would be that the stock gauges for the 2.9L aren't reading correctly in the upper RPM ranges when attached to the 4.0L, either from gearing or just the mechanics. You could verify correct RPM calibration in the upper ranges with an engine analyzer that has an RPM readout.

A bigger MAF isn't going to help MPG. IF it does, it's usually because it's screwed with the ECU and made for a lean fuel condition. Not the way I'd want to save gas, personally. I say keep the stock MAF sensor and housing unless you go with more involved engine mods. Get the CAI instead.


You're saying two different things, if you want MPG, your best bet is to never even go near the rev limiter. Keeping the motor under ~2500-2600 RPMs (max torque) will get the best MPG, same goes for highway driving, usually 2200-2300 is even better, though sometimes that means ~55 mph with big meats, and isn't enough RPM for inclines and hills.
 




jd4242

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JD4242...
I now know that the tranny is the issue. The tranny isnt malfunctioning, its doing what its supposed to do with a shift kit installed. But what is the best way to fix it? I was concerned that it was something wrong with my swap but it looks like this engine just doesnt rev as high as the 2.9.

Would adding a chip with a higher rev limiter fix the issue and who makes a good chip or tuner. As stated previously, its big heavy and slow...Im ok with that, just want to fix the rev limiter issue and any MPG gain is a huge plus!

They dont make a chip.adjust the vacuum shift module on the side of the tranny so it shifts before redline!!
 




59Bisquik

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Im not trying to say two things. I do want to get good mileage and when on the freeway I try to keep it a bit slower and keep the RPM's down and try to keep the torque convertor in lock up. Again, I realize its no speed racer and with its super soft suspension, I try to keep it on the slow side anyway.

However, when the need for acceleration presents itself...getting on the freeway or passing a big rig on the grades for example, it will require some WOT action. I know this kills mileage, but is a necessary evil when driving a B2!

I had read that the bigger MAF helped out alot once the CAI was installed since the MAF was then the source of restriction. But it sounds as if it might not be the right answer for me. A custom CAI is on the list and it might even be ram air with a scoop on the hood. (this will be instead of a snorkel since I want to raise my intake height and dont want to hydro another engine and think snorkels look horrible).
 




59Bisquik

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There is no chip that will raise the rev limiter? I have had one on my old Thunderbird that raised the limiter and removed the speed limiter etc.

Who makes an adjustable vacuum module? This might be less expensive anda better choice than a chip.
 




jd4242

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Not for a 91.you might have one,just a pia to get to.you dont want a stock 4.0 going over 5000 anyways,your float the valves.i can send you links when i get home.
 




59Bisquik

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How can I tell if I have an adjustable one? BTW...the tranny was originally an 86... I didnt use the 91 tranny since it was in poor shape and a 2wd.
 




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