Hesitation under load, only when warm

[donutbob]

Hi guys,

So... I start the car, let it warm up for a few minutes and get going, car runs fine. Car warms up, according to the gauge, after a few more minutes, I can go WOT and she runs strong, no problems... After about 20minutes of driving is when it starts to get "unhappy". She free revs in neutral as if nothing is wrong, but...

Here are the symptoms:

Under load, under 3500rpm, she is very difficult to drive, bucks - hesitates - misfires, just generally unhappy and difficult to drive. Temp normal, oil pressure normal, no check engine light.

Cruising above that rpm, 4-6k, its fine, no symptoms.

WOT above that RPM, is also ok, until about 6500rpm, where i feel a little misfire, as if the traction control is intervening.

I borrowed a diagnostic tool to see if there were any codes stored in the ecu, none. Fresh gas, clean air filter. I don't know where to start.

Any ideas? Its a 1991, 130k, all service up to date.

Thanks in advance

 

[Briank]

My guess is one of your O2 sensors is going bad but its not bad enough to through a code. What kind of diagnostic tool did you borrow to check for codes 91's don't have a diagnostic port.

 

[donutbob]

Heh, I borrowed some sort of jumper i plugged into a two prong connector under the glovebox. Was this procedure wrong? The CEL on the dash threw no codes with the key in the on position, while this "tool" was plugged in.

I shouldve asked if this step was correct, my friend has a generic book showing all manufacturer diagnostic codes, and a how to access section.

 

[Briank]

Yes a jumper is what you needed most of us just use a paper clip.

{EDIT}

One of the tests for a bad O2 sensor is to accelerate from 1200 RPM in forth gear for at least 5 seconds and then cost for at least 5 seconds and then repeat and see if you get a code.

 

[donutbob]

Thanks Brian,

I've been working on the suspension, I'll try this tomorrow. If I can't get it to trigger, new o2 sensors anyway? I hate to throw money at the problem, I really hope she tells me whats ailing her.

Don

 

[Briank]

I would not replace the o2 sensors unless you get a code you might also look into the main relay, lots cheaper to replace, can even be patched up if you are handy with a soldering iron and is common problem.

 

[ak]

Have you checked your coils? I had similar symptom when I had water in the rear bank coils.

-ak

 

[goldNSX]

I'd vote for O2-sensors or bad rear bank coils too.

 

[donutbob]

Well, finished up the spring shock install, and tried to get that CEL to trigger, no luck. Is there a way to check the o2 sensor itself?

Thanks again for the leads, out of curiousity, with this information, what do you think a dealer would do?

 

[DRIFTER]

My car had the same symptoms. check the spark plugs:smile:

 

[Briank]

Thanks again for the leads, out of curiousity, with this information, what do you think a dealer would do?

Without a trouble code most will not have clue what to do other than a useless injector cleaning which seems to be the best way they have come up recently to make money.

 

[donutbob]

Will do, I don't think I'll have time to attack this till the weekend, but will report back after, I appreciate the leads.

My car had the same symptoms. check the spark plugs:smile:

 

[donutbob]

Ok, found it!

I didn't get a chance to work on the car till today, but luckily the problem revealed itself right away.

Spark Plugs. Good call DRIFTER!

The top of my plugs were corroded, to the point the metal was brittle. The coil pack was also showing some corrosion. So, I got some new plugs, cleaned the inside connector on the coilpack as good as I could, and put it all back together.

I'm thrilled! NO more hesitation, and it idles much smoother now as well, I didn't even think it wasn't idling well. I don't understand the sudden need for the plugs, but just as an FYI, maintenance is up to date, the only possible difference is the car is pretty much only a de car, lately. Parked for weeks to months at a time, then driven at an event, fluids and such checked and reparked.

Thanks again for all the help...
Don

 

[tuned-r]

Don,

Which plugs was corroded ? I believe it must be from the rear banking.

Water tend to splash into this area during wet driving or rain (stationary).
I had plug corrosion problem several times and now I have decided to replace all 6 coils becos :
1). The rubber caps dont seal well from water/moisture
2). Micro crack on top of the coils that can absorb moisture

Check this for future re-occurance.

Cheers

T-R

 

[donutbob]

Mine was one in the front bank, the worst, and two in the rear bank. How soon after changing plugs did you re discover the corrosion? Did you have similar symptoms as I?

Just as another FYI, I've never clean my engine bay in any manner, but I do keep the cover off and it's always been off since new.

Don,

Which plugs was corroded ? I believe it must be from the rear banking.

Water tend to splash into this area during wet driving or rain (stationary).
I had plug corrosion problem several times and now I have decided to replace all 6 coils becos :
1). The rubber caps dont seal well from water/moisture
2). Micro crack on top of the coils that can absorb moisture

Check this for future re-occurance.

Cheers

T-R

 

[JohnJW]

Have you checked your coils? I had similar symptom when I had water in the rear bank coils.

-ak

yeah this is my bet.......my 92 did same exact thing.

O2's are fine

they would throw off a/f but would not cause a misfire

 

[Larry Bastanza]

bunta,

Make sure your coil covers are on correctly. The rubber seal must be on top, on the intake side of the coil cover. If they are swapped, it is likely to cause your problem.

Regards,
LarryB

 

[garyn]

If you find water on your coils, there may be a pool of water built up in the spark plug well. Make sure to clean or blow out the water before taking out your spark plug as it may drain into the cylinder. My cover was reversed so water was leaking into the rear bank. I hear sos has covers with gaskets on both covers.

 

[donutbob]

I considered that, but aren't the middle nuts/studs coming out of the valve cover offset to one side, making it impossible to flip? I'll check again regardless.

bunta,

Make sure your coil covers are on correctly. The rubber seal must be on top, on the intake side of the coil cover. If they are swapped, it is likely to cause your problem.

Regards,
LarryB