94 explorer, just swapped in a m5od transmission. The truck starts up and runs, but it boggs down at high rpms. It shifts good, has good low end power when I'm working the clutch, but I can't coast at a constant speed or accelerate for too long without losing all throttle response. The only way I can drive the truck is to keep my foot pressed slightly on the clutch pedal at all times. This revs up the motor and allows me to accelerate/cruise without stalling.

Does anyone know what this could be? The truck drives the same with either a manual or automatic computer so I ruled that out. My vacuum lines have all been replaced or plugged so I don't think I have any air leaks under the hood. I had to buy a new y pipe and I broke two of the four studs removing the old one. I couldn't drill them out so I bolted up the y pipe with just one stud per side. Also the y-pipe to converter flange studs are missing a nut at the moment. So there's definitely an exhaust leak that must be fixed, but would that cause these symptoms?

The oddest part is that ever since the transmission swap my check engine light wont flash codes anymore. I'm using the same method that's always worked for me...jumping the ecu test circuit with a copper wire and counting how many times the CEL flashes. Sometimes it will flash really fast like it's about to work, but then it never does anything. So I'm unable to check for codes

 

Some (all?) O2 sensors reference to ambient air so if there's an exhaust leak in the area of the sensor, the reference is screwed up. This might be your problem or at least a contributing factor.

You should get your hands on a schematic for a '94 and see where the clutch pedal position inputs are used. For my '98, there is an input to the computer from the clutch pedal position switch that is used to adjust air/fuel ratio and idle speed. The other two contacts on the clutch pedal position switch are used to cancel cruise control and to inhibit starting. Based on this, I would say that the computers aren't the same, but who knows what the effect is?

With your foot on the clutch and the motor revving, it sounds to me like your clutch is slipping.

 

Thanks for the info. During acceleration I can hear a faint whistle which sounds like a vacuum leak, but I'm assuming it's coming from the exhaust. I'm currently drilling out all the old seized exhaust studs and replacing them with stainless steel nuts and bolts to take care of the leak.

I wasn't using the clutch pedal switch because I have it bypassed so the truck will start in any gear. I'll try plugging it in while using the manual computer and see if that changes anything.

The clutch, pressure plate, slave cyl and throwout bearing are all brand new, so I wouldn't expect the clutch to be slipping. There's about an inch or two of pedal play before the clutch disengages which seems normal. It's possible I still have some air trapped in the system, but I don't know how that would affect my throttle.

 

Ok the truck still has issues. I sealed a leak in the air intake tube and the exhaust. Still Boggs down upon hard acceleration and the CEL pops on. When it boggs down I have to hit the clutch and rev up the motor in order to gain any speed. The truck won't redline because it boggs but when I press the clutch the CEL usually immediately goes away. But when I go to check the codes, no codes flash at me. I'm thinking about replacing the plugs, wires, fuel filter and o2's. I'm just trying to narrow down what it could be if the CEL is on intermittently while I'm driving but doesn't flash any codes.

 

Which PCM do you have installed currently, the manual or automatic?

Doing the standard tune up items is not a bad idea since it sounds like you plan to keep this truck awhile.

Do you have an OBD-I scan tool that you can use to pull stored diagnostic codes and enable the KOEO and KOER tests? With such a scan tool you should also be able to perform a cylinder balance test which might help identify an issue.

-Rod

 

I've tried the auto PCM and a manual PCM from a 94 explorer. There's no noticeable difference between the two different computers.

I do plan on keeping this truck. I swapped in the 5 speed to extend the life of the truck since the auto trans had almost 200k on it.

Before I spend too much money guessing on parts I want to figure out the codes. Usually I jump the pins on the ECU test under the hood and count the amount of times the check engine light flashes to figure out the codes. This method hasn't been working since the swap, but it did work once randomly and the code was 998, "hard fault present". What exactly does this indicate?

 

The only extra nugget of info I find in the factory service manual for code 998 is that this hard fault is related to the FMEM Mode. FMEM is Failure Mode Effects Management and this suggests that one or more sensors are giving screwy readings, or no readings at all, and the PCM is not sure how to deal with those readings which could cause a failure of the emissions system (catalytic converters primarily) if left unrepaired.

If you can find an Ford OBD-I (EEC-IV) scan tool with datastream mode that might be quite useful in detecting which sensor or sensors are not functioning properly.

-Rod

 

Finally got the codes. I'm getting 998 (O hard fault present), 13 (R rpm not within self test lower limit), 18 (R SPOUT circuit open), 23 (O,R throttle position sensor out of range) in that order.

 

Those codes are bogus. 91 & newer fords use 3 digit codes.

BTW, have you ever checked fuel pressure? If it's low it will cause it to bog down.

When I had a warped passenger side exhaust manifold on my 87 2.9 the only way I knew it was from the loud ticking noise. It didn't effect the running at all. If you do have a slight exhaust leak it's probably not the problem.

 

Well those are the only codes I got from the test and they are all I have to help point me in the right direction.

 

You might want to do a goggle search on 'innova 3145 + amazon" click on the link with the two hundred and something user reviews of the 3145. The innova retrieves stores & displays the trouble codes, is very easy to use & is only 25 bucks.

It's a shame the 18 code isn't correct as it would be a perfect match for your bogging down problem. Getting the correct codes should be a big help with this

 

I don't see why the self test codes wouldn't be reliable. If these codes are accurate, what should I check? Seems like the SPOUT wiring could be damaged. Does the TPS voltage code mean that mean the TPS is bad or is that code just being triggered by a different problem?

 

At higher rpm's the PCM advances the timing to around 30 BTDC. If the spout circuit was open the advance would stay at around 10 degrees.

As for the TPS, when you are signaled to stomp on and release the throttle is when the TPS voltage check happens. Voltage should go from 1 volt at idle to 5 volts at full throttle.

 

I feel like I'm in way over my head. The truck is still in "limp mode" and I can't figure out why. I haven't done any voltage or fuel pressure tests, mainly because I don't have the tools for that. I've replaced the TPS, CPK, ICM, cleaned the MAF and IAC, but this made no difference. I'm almost certain that it's a vacuum leak (even though I've combed through them numerous times and found no visible leaks) or an electrical short. Another note to add is that the truck occasionally cranks but won't turn over, usually after it's warm.