93 Abrupt Throttle Response

[PropellerHead]

Hi Explorer experts! I'm the original owner of a 93 Explorer, manual trans, with 214k miles. Great truck and has been very reliable with no significant quirks. Until recently.

After a dealer 60,000 mi service (with fuel system cleaning, new clutch and slave cylinder) has developed a very abrupt throttle on/off response. It's most noticable when trying to maintain a steady speed on level road. Very gently easing off the throttle results in a "kerchunk" as all the drive line lash is suddenly taken up transitioning from pulling to compression braking. Very gently adding throttle results in another "kerchunk" as driveline lash is taken out in the other direction.

It's as if the engine RPMs want to drop too fast. I have noted that the symptoms only occur when the engine has warmed enough to get the temp guage into the bracketed range. When the engine is cold, there is no problem and on/off/on throttle respose is very smooth.

'Seems that the problem might be manifesting only when the computer goes into "closed loop" mode.

Not having an opportunity to leave it at the dealership for them to sort out, I've been trying to diagnose this myself.

I've replaced the IAC and TPS with new aftermarket units with no change in symptoms. Bought an OBD1 code reader, and it showed a continuous code 157 (MAF low voltage). So, I pulled the MAF sensor, checked the connectors and cleaned the sensor.

Disconnected the battery to clear the code. Reconnected and drove for a day. Now, when doing the KOER test, it shows a 411 code (can't control RPM) and seems to be unable to drop the RPMs as much as it used to during that test.

The 411 situation may be a second problem. For now, I'm shooting to get the abrupt throttle response cured as it's very hard on the drive line.

Any leads will be greatly appreciated!

 

[MadExplorer93]

Check all vaccum lines for a break in them expecially the FPR line = fuel presure regulator which it has a small plastic line that is plug into the back of it.

Does the engine idle around 1200-1500 in morning or when warm or is it low 500-600, now 500-600 is normal when warm and when warming up, but if it is surging or wants to stall this can cause an issue.

But check all vaccum lines, see of FORD forgot to plug the FPR line back in or if they broke it as they become brittle over time and are easy to break.

The FPR is right on top of the left side of the engine and beside the FUEL RAIL, you can't miss it as its the only thing there that has a Thin Plastic Vaccum line on it which runs to the vaccum TEE on the plenum.

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The other possibility is you may have a BAD COIL, but that will need to be tested which you can do with a OHM meter for resistance when engine is COLD and then when engine is Hot to see if there is a major difference.

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Also does your 1993 have a EGR valve on it, if so then check the vaccum to it as well, and at times if the EGR valve is no good then it can cause a rpm/rough idle issue as well.

 

[PropellerHead]

Additional Checks

Thanks MadExplorer93 for the tips. Here's today's findings:

I checked all the vacuum lines, disconnecting them one at a time from the manifold tee and sucking on them. They all held vacuum well, except the line having the small tee over on the passenger side that goes to the A/C and air box. The tee leg to the A/C seemed to seep a little (more on that later).

This engine apparently doesn't have an EGR valve. I did remove the PCV valve and verified that it rattled when shaken.

I started the engine cold (72F outdoor temp), and it idled up to 1500RPM and gradually over the next 2 or 3 minutes idled down to about 900RPM. Once there, it oscillated between 900 and 1000RPM.

I let it come up to operating temp (gauge within the bracketed range) with an occasional throttle blip. With each blip, the RPMs would pause around 1500 before coming down.

Then, wwitched the engine off. Upon restart, RPMs held briefly about 1900 before settling down to an almost steady 700PRM. By this time the engine was warmed up.

Did another KOEO check and there were still no codes. Did KOER check and again got the 411 (cannot control RPM during KOER low RPM check).

Now, about that vacuum line to the A/C and air box: To get a baseline vacuum reading, I connected my trusty vacuum gauge to one of the blanked-off manifold ports and read a steady 19" of vacuum at 700RPM. Then, at the manifold ports, I disconnected the line to the A/C and airbox and blanked off that port. The gauge showed no change in vacuum.

Just to see if, by chance, it made any difference in drivability, I took it for a spin. The abrupt throttle symptom was still there. That seems to indicate that the seeping vacuum line to the A/C isn't the main culprit - but could be a contributor, depending on what the A/C branch is supposed to be doing.

I haven't checked the coils yet but will at next opportunity. This Explorer still has the factory plug wires and I'll probably replace them just to rule them out.

Also, the book that came with my Innova code reader says to be sure that the computer can control ignition timing. Any clues on what angle works best for viewing the timing marks?

Many thanks!

 

[briwayjones]

This Explorer still has the factory plug wires and I'll probably replace them just to rule them out.

WOAH! If you have 214,000 miles on there and the original plug wires on there change those bad boys out. I would do that first. The wires should be changed every time you change the plugs and could very well be your problem.

 

[Theguy155]

did you ever figure this out? my 93 5 spd manual X is having the same problem