Backfire at 4000 rpm???

[Glenn Biggs]

Maybe someone can help. I have a 1992 4x4 EB Explorer (auto trans). When entering a freeway or passing, I accelerate, trans drops to 2nd and when reaching 4000rpm it starts to backfire, as it shifts to 3rd, rpms drop and the backfiring stops. It seems to do it more with med grade gas than the premium. I wonder if the computer can't keep up with the timing? Maybe I need a Superchip? Has anybody heard of this problem before? I have made the following engine mods; K&N fltr w/air box mod, bigger MAF and TTB, Pacesetter headers, high flow cat w/flowmaster catback, Jacobs wires, Bosch platinum spark plugs gapped at OEM specs, RV cam on engine rebuild. Any replies will be greatly apreciated. Thanks.

Glenn Biggs

 

[Alec]

Any chance the problem is with the cam? Maybe the cam is lobed for lower RPM's and doesn't trigger the valves well above 4000 RPM's. I dunno, I figured it was worth the guess. If it happens less with higher octane, go ahead and use the higher octane gas. With all that other stuff you've done, your engine will run much better. You might want to think about getting a custom burned Superchip. A "stock" Superchip wouldn't work well with all your mods.

You're truck sounds pretty fast. What was the gain like from the cam?

 

[Glenn Biggs]

Don't know. My engine died one day and took it in for a rebuild. I was planning on boaring the block and everything. But cracked heads shot down my plans. $300 each head. So the only mod I did inside the engine was the RV cam. Since I had someone do the job it cost me $4000 for a new engine. So I have'nt kept track of before or after installs on mods. Thanks for replying though.

 

[Chris Jella']

Glenn,
I'm having the same trouble at around 4500 RPM on my 93. Mine is all stock. Mine sounds like it is coming from the engine compartment on the passenger side. If you gag it from a stop or make it downshift to get to that RPM range, I usually get 2 or 3 pops and then it upshifts and it stops. I also notice a slight ssst ssst ssst sound at idle and wonder if an exhaust or header leak might be to blame, but so far I can find no sign of leakage. The odd thing is all other aspects of drivabilty are fine. Gas mileage, power and such all seem normal. I just pray it's not a cracked head or something really bad. If I find something, I'll post it. Good luck on yours!!
CJ

 

[SteveVB]

Glenn, what cam did they put in? Just curious.

Assuming the mechanicals are ok. I would check the fuel system, check the pressure -may want to drive it with a gauge see if it stays around 40psi , could be the regulator, or an injector thats stuck.

Also check the TPS (throttle position sensor) it can cause all sorts of problems if its not sending the correct signals.

Good luck.

 

[Rhett]

What gear ratios do you have to get 4000+ rpm? That seems super high to me.

My Explorer _never_ gets much above 3500 rpm even when I really nail it. I have 3.27 gears.

 

[murrajm5]

You might try the plugs because I had the same ones go bad after only a year. Replaced them with the +4's and no problems since.

 

[914x4sport]

Originally posted by Rhett Browning
What gear ratios do you have to get 4000+ rpm? That seems super high to me.

My Explorer _never_ gets much above 3500 rpm even when I really nail it. I have 3.27 gears.

 

[914x4sport]

Originally posted by Rhett Browning
What gear ratios do you have to get 4000+ rpm? That seems super high to me.

My Explorer _never_ gets much above 3500 rpm even when I really nail it. I have 3.27 gears.

Are you sure? My stock 91 sport (auto)can get up around 5K when I really put the hammer down, no problem. And if you have a standard you should be able to push it as far as you want.....maybe not??

 

[Rhett]

I'm sure. My truck never goes above 4000 rpm ever, and I have nailed the gas from a stop and it only hits the high 3's (3800 rpm or so).

Interstate cruise for me is 2050 rpm @ 70 mph. 2200 rpm @ 75 mph.

I have 3.27 gears and auto trans. Maybe if you have 4.10 gears it will cause higher rpm? I am still curious as to how you're getting 4000 or 5000 rpm on the 4.0.

 

[Glenn Biggs]

I have 3.73 limited slip. In second gear it will reach 4000 rpm, point where it shifts down to 3rd.

 

[Alec]

I have a 96 4.0 w/Auto and 3.27. Stock, it could get up to around 4500. With my Hypertech and other mods, it frequently went about 5100. I haven't watched it closely since I got the blower installed, though. That was just today, so I'm still having fun with it.

 

[Glenn Biggs]

How much for the blower?

 

[Alec]

$3500 installed