Part-Throttle Hesistation

[gsr20det]

My 1990 SHO has been a great car for the couple months I've had it, but I've run into a problem lately.

When pulling away from a light, the car has developed a hesitation at part throttle. The more I go into the throttle at low RPM, the worse it is. If I barely tip into the throttle and ease into it more, there's usually no problem. Try to pull away quickly and the car hesitates, sometimes even backfiring in what sounds like the intake (not out the tailpipe). I checked and the car is not showing any codes. Plug wires are a little over a year old, as are the plugs.

I just replaced the mid-pipe (one of the cats was wasted!), and the car picked up quite a bit of power from it.

I also am not getting very good gas mileage. I average 19-20mpg in mixed driving, and on 2 trips to San Fran from Los Angeles, I also got no better than 22 mpg with an average speed of 85.

Anyone had similar issues? Any suggestions welcomed.

 

[SHOguy 92]

Bad gas mileage.... my guess would be this problem could be partially o2 sensors, but I'm thinking thats not all of it. Maybe MAF, check you plug wells see if there is oil in there. If so check into new plugs, wires, and valve cover gaskets.

 

[gsr20det]

Thanks for the speedy reply.

I checked the MAF by unplugging it and driving around, and still get the exact same issue. Replaced the valve cover gaskets a while back, plug wires are dry (replaced the front main seal and timing belt at the same time, too - gotta love picking up parts at Autozone on a sunday!! Can't do that on the Nissan).

O2 sensors look like they are stock, car has 140+k on it. May replace them next.

I guess I'll swap out the wires, plugs, O2 sensors if there seems to be nothing else that it could be. The lack of any codes showing up has be guessing it may be the plugs and wires since the O2 sensors woudl throw a code, no?

 

[Slo-Sho]

Has the timing belt ever been replaced?

 

[gsr20det]

Yup, replaced the timing belt last summer. Did the 120K in August of last year.

 

[Sexymeatball]

could be any number of things. For me it was pretty much everything (O2 sensors, MAF, plugs, wires). Replaced single items from time to time which temporarily seemed to alleviate the issue, but it didnt get rid of it. It only went away when i bit the bullet and bought new plug wires and plugs. Good luck.

 

[gsr20det]

Bit the bullet!

Just ordered OEM wires and plugs, put them in tonight and see what happens.

Thanks everyone for the replies!

 

[Yamaha V6]

Did you replace both cats? Sounds like possibly a plugged cat - the harder you get on it, the worse it gets, to the point of sputtering / backfiring?

 

[gsr20det]

Yea, both cats were replaced with a single cat in a custom y-pipe.

If I jump on the throttle and stay in it, it will continue to hesitate. If I ease into the throttle and eventually go full throttle it stays fine. If I jump on the throttle and it starts to hesitate, then I ease back out of it and it will clear up, then I can ease back into the throttle and it's fine again. Gas mileage is never more than 20 or 21 mpg, freeway or mixed driving. Car throws no codes also.

Cliff notes: Jump on throttle, car hesitates. Ease into throttle, car is fine. Can get rid of hesitation by laying off gas then easing back in.

I get the wires/plugs tonight and will see what happens I guess!

 

[betterman]

Clean your MAF sensor, common problem on the SHO that will cause hesitation.

 

[gsr20det]

Purrs like a kitten, pulls like a tractor!

Well, it was a long day, but worth it!

Took the intake manifold off as I figured it would be the easiest way to make sure everything is alright, glad I did. Found the runners and the head to be all gunked up with all kinds of oil

SO I completely disassemble the intake runners and clean them out with carb cleaner and a brass brush. Then I hit them with some Gunk engine cleaner and a hose and let them dry out. Ran some more carb cleaner though it one last time and considered painting them white, but decided I'd keep them looking stock.

Swapped out the wires and plugs, also sprayed the MAF with some electric parts cleaner and fired it up. Took it a second to get running (had to clear the left over carb cleaner I guess), then settled into a nice idle. Revved it and it was as clean as could be, no stumble, no backfire. Took it for a spin and the car flat out yanks now. I used to have to ease into the throttle, getting a stutter at any time below 3500 if I was at full throttle, or if I even jumped on the throttle too quickly (like if I was stopped on a hill in San Francisco and tried to pull away). Now it pulls hard right from the get go :****:

Thanks to all for their opinions. Amazing what a proper set of wires and plugs will get you!

Now if the tranny in my race car could get better with some plugs and wires...