1988 Hesitates when raining

[LostinDartmouth]

Hi there 4rnrs.

I have a 1988 4runner sr5 EFI with a strange issue...About 4 to 5 minutes after the rain starts the engine starts to hesitate in the mid range. Idles fine, excellerates fine when you stomp on the pedal, but when trying to hold a steady speed or put a medium load on the engine it starts to hesitate/buck.

5 minutes after the rain stops, the problem stops.

Doesn't matter what gear I'm in....only happens when raining, not snowing.

Thoughts are appreciated.


FYI- All new plugs, rotor and cap, sparks last fall and are in great shape.

Thanks Paul

 

[cometobanks]

,moisture in distributer?

 

[LostinDartmouth]

That was the first place I checked...found it to be 'bone-dry'...

 

[turboSR5]

Air flow meter perhaps?

Edit:

Probably compeletely unrelated but over the weekend I was checking on the heater hoses and in doing so I knocked the vacuum hose off of the EGR diapraghm (larger hose on the very bottom). It ran fine, Idled fine but when the EGR went to open up around 2,800 to 3,000 RPM it started surging. As soon as I realized it was at EGR opening speed I found the problem.

 

[LostinDartmouth]

Hey turbo....Air flow meter?

I've replaced the mass-flow sensor with a spare that I know works, but the problem still persists when raining...very strange.

Options could include:
1) there could be a grounding/electicial short issue in the engine compartment when the water sprays up from the wheels and wheel wells, and/or
2) the forward momentum of the truck, along with the rad fan, are blowing water across the engine...

Would water spary near the alternator cause the problem?

 

[turboSR5]

I'm pretty sure this is an electrical problem of some sort that is aggravated by moisture. What makes me think AFM is you are sucking wind at the front of the grill and it flows right on through the AFM as the first point of contact. What I would do as a test is record the resistance at the pins on the AFM when it is running fine. Then go for a drive on the next rainy opportunity and then measure resistance again once you get the symptoms you described above. Wouldn't hurt to do the same at the TPS also. I doubt your altenator is to blame but I have been wrong more than once.

 

[LostinDartmouth]

Thanks turbo..I'm assuming the airflow meter is the same as the mass flow sensor...ok?

Your assumption of an electrical issue could be right. I filled in a rust hole in the driver's side wheel well that was directing tire water to the fuse box and wiring harnesses.

I also did a spray bottle test when the engine was running...spray a bit and then drive for a bit...repeat the process until something crops up...

Sprayed the disbrituor cap and seals, the TPS, and the ignitor....no change in engine. However, when I sprayed the fuel injector seal on #2, the engine faltered. Will try a sealant on it (gasket sealer or dielectic grease) on the external seal and then wait for another rain storm...

My gut instincts tell me I need to keep looking.....

 

[LostinDartmouth]

Solution Found

Hey Team:

Finally found the problem:
The wires to the oxygen sensor.
Two of the 4 wires were cracked allowing moisture to short the signals to the ECU which makes the truck behave like it is on mexican beans.

I'm surprised that such small wires (18 gauge?) underneath the body can cause that much of a problem with the engine.....

I use a hose to spray sections of the engine to simulate rain. As soon as I sprayed the wires the engine nearly stalled.

I tried to wrap them with electrical tape, but even that allowed enough moisture to still affect the engine, so Dipped the wires in liquid plastic several times until coated.

It has been 5 weeks since this fix, and the engine runs like new. Rain, puddles, mud, foggy atlantic days...

Who would have thought?

Paul

 

[turboSR5]

Excellent, that took some determination to figure out.

 

[KidVermicious]

Thanks for sharing. Mine does the same thing and it's been driving me nuts trying to find it. I'll check those wires.