Hello, and thank you guys for all your excellent advice. First off, I'll admit to more than a little embarassment posting here for help - I'm a full time tech and former Ford Master tech, as well as ASE and all that. But! I'm failing here to diagnose a problem with my 89 SHO. Car is a daily driver with somewhere north of 200k on it (speedo cable broke and I haven't found a replacement yet). It's running poorly at low RPM as my title states. Working with the search fuction, I've tried several things, but nothing seems amiss. I am having a severe hesitation or stumble when taking off from a stop, and some misfire under heavy throttle, but only until about 3000 RPM. My plugs are new Autolites, wires routed correctly. No damage to wires and the resistance is within spec. I'm getting no codes from the ECM currently, but I did have a code 24 for the intake temp sensor, which I have replaced. Any suggestions? I'm pulling what little hair I have left at this point.
Running poorly at low RPM
- Thread starter Philbert
- Start date
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Eric B
New Member
Mine was misfiring under 3000 rpm with medium or heavy throttle, cleaned MAF with electrical cleaner and was fixed.
I had another problem with rough idle which turned out to be a leaking fuel pressure regulator, I pulled the vacuum line and there was fuel, it was leaking past the diaphragm. Again, no codes.
I had another problem with rough idle which turned out to be a leaking fuel pressure regulator, I pulled the vacuum line and there was fuel, it was leaking past the diaphragm. Again, no codes.
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Thanks for the reply Eric, but I've already cleaned the MAF and my fuel pressure regulator seems to be working properly, I checked fuel pressure at the rail and it was within 2 psi of the spec - 44 PSI static pressure and 30 running.
shoon
cliTaurus
Has it just started acting like this or has it done it recently after service? Going to throw some stuff out there, hopefully you can go off of something here:
Guessing you've already gone over it and verified all vac lines are connected and properly routed.
What were you using as a reference when hooking up spark plug wires? There are a few publications out there that show incorrect routing (chiltons maybe haynes too I believe) Double check routing with this
Try unplugging the MAF and see if idle improves; if it does likely faulty MAF.
Pull the IAC / inspect and clean it.
Not sure if you have a tool to verify the presence and strength of spark, but hooking one of those tools up to the end of each one of your wires would also be a good way to verify all cylinders are getting spark right up to the ends of the wires.
How did the old spark plugs look, and whats the exhaust like coming out the end? rich / lean ?
Guessing you've already gone over it and verified all vac lines are connected and properly routed.
What were you using as a reference when hooking up spark plug wires? There are a few publications out there that show incorrect routing (chiltons maybe haynes too I believe) Double check routing with this
Try unplugging the MAF and see if idle improves; if it does likely faulty MAF.
Pull the IAC / inspect and clean it.
Not sure if you have a tool to verify the presence and strength of spark, but hooking one of those tools up to the end of each one of your wires would also be a good way to verify all cylinders are getting spark right up to the ends of the wires.
How did the old spark plugs look, and whats the exhaust like coming out the end? rich / lean ?
Hey Shoon, it seems to have slowly been getting worse over time, such that I didn't notice it for a while. I have been keeping up with maintenance myself, and I'm pretty sure that the vacuum lines are all good. I can't find any leaks with starting fluid anyway.
Plug wires I routed according to the way they were in there already, and I did double check with the routing in the diagram you linked, they seem to be correct. MAF unplug does nothing for the better, seems to make it slightly worse, as does removing the spout shorting bar.
The IAC doesn't appear to be out of sorts, idle dies when it's disconnected, and when I switch on the AC it idles up properly.
Spark tester shows ok at 20kv marks, peters out before the 50k mark, but I can't get access to test the rear ones properly - so coils should be good, and since the wires in back tested ok for resistance I'm assuming that the spark strength to those is ok as well. I don't have access to an oscilloscope, but I feel confident in my tests that the secondary ignition is good. Old plugs looked ok. Showed signs of normal combustion, maybe slightly lean? I had replaced them about 40k previously, but since they aren't too expensive I started there and replaced them last week.
Exhaust doesn't smell particularly bad. It's definitely not rich, but I don't have a sniffer available to get readings on it or anything (we're not in a smog check area). It's a bit hard to get any real readings when it's having problems, since it's a manual and it idles ok but runs poorly under load at low speeds. I had a known good TPS in my toolbox so I threw that at it hoping for the best, but nothing changed.
Plug wires I routed according to the way they were in there already, and I did double check with the routing in the diagram you linked, they seem to be correct. MAF unplug does nothing for the better, seems to make it slightly worse, as does removing the spout shorting bar.
The IAC doesn't appear to be out of sorts, idle dies when it's disconnected, and when I switch on the AC it idles up properly.
Spark tester shows ok at 20kv marks, peters out before the 50k mark, but I can't get access to test the rear ones properly - so coils should be good, and since the wires in back tested ok for resistance I'm assuming that the spark strength to those is ok as well. I don't have access to an oscilloscope, but I feel confident in my tests that the secondary ignition is good. Old plugs looked ok. Showed signs of normal combustion, maybe slightly lean? I had replaced them about 40k previously, but since they aren't too expensive I started there and replaced them last week.
Exhaust doesn't smell particularly bad. It's definitely not rich, but I don't have a sniffer available to get readings on it or anything (we're not in a smog check area). It's a bit hard to get any real readings when it's having problems, since it's a manual and it idles ok but runs poorly under load at low speeds. I had a known good TPS in my toolbox so I threw that at it hoping for the best, but nothing changed.
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shoon
cliTaurus
Going to throw some more stuff out there to check:
Pull your upper timing cover and inspect for orange particles (usually powdered steel that flake off woodruff key and corrode inside covers) And also condition and tension on the timing belt while your in there.
http://wikisho.com/krazgeo/CrankCancer.html
Might be a case of crank cancer causing the timing to retard. Another way to check would be to crack your crank bolt loose and attempt to spin the pulley back and forth (clockwise / counterclockwise). If the pulley spins or has play while the crank remains stationary you have a problem.
Are there any abnormal noises while running? Any issues with the tachometer working? Hard starts?
Next possibility is cam sensor; when it goes bad it usually causes hard starts and misfiring.
-Ensure good continuity between ground and the DIS bracket (surge tank) As this bracket is isolated form the intake and gets its ground connection through a jumper wire which sometimes corrodes / goes bad.
Pull your upper timing cover and inspect for orange particles (usually powdered steel that flake off woodruff key and corrode inside covers) And also condition and tension on the timing belt while your in there.
http://wikisho.com/krazgeo/CrankCancer.html
Might be a case of crank cancer causing the timing to retard. Another way to check would be to crack your crank bolt loose and attempt to spin the pulley back and forth (clockwise / counterclockwise). If the pulley spins or has play while the crank remains stationary you have a problem.
Are there any abnormal noises while running? Any issues with the tachometer working? Hard starts?
Next possibility is cam sensor; when it goes bad it usually causes hard starts and misfiring.
-Ensure good continuity between ground and the DIS bracket (surge tank) As this bracket is isolated form the intake and gets its ground connection through a jumper wire which sometimes corrodes / goes bad.
Thanks for your continued help, Shoon. I'm sure that my ground is good on my DIS module, as I made a separate ground strap for the surge tank while trying to diagnose a previous problem 
It's hard to hear any noises while it's running, because the alternator bearings recently started to go, and are loud, but looking at the descriptions of the crank cancer I'm seeing many familiar signs :scared: I think I'll try and take it loose to check tomorrow at work.
It's hard to hear any noises while it's running, because the alternator bearings recently started to go, and are loud, but looking at the descriptions of the crank cancer I'm seeing many familiar signs :scared: I think I'll try and take it loose to check tomorrow at work.Well, it is as stated. My crank sprocket was WAY off the keyway, having walked out about 25 degrees. Crank bolt was only finger tight when removing it.
Does anyone think it's worth it for me to repair this and do a 60k? Car is in rough but running shape, and needs an axle to really be in drivable condition again, but that's not counting all the ancillary stuff like door handles and locks, rear defrost grid, etc that's not in proper working order.
PS - sludge is from me cleaning off the oil residue - crank seal is starting to leak. I got it off after picture
Does anyone think it's worth it for me to repair this and do a 60k? Car is in rough but running shape, and needs an axle to really be in drivable condition again, but that's not counting all the ancillary stuff like door handles and locks, rear defrost grid, etc that's not in proper working order.
PS - sludge is from me cleaning off the oil residue - crank seal is starting to leak. I got it off after picture
shoon
cliTaurus
Thanks for following up! Glad you found the cause of the problem; but at the same time I am sorry it is crank cancer :cry:
How does the crank pulley look?
chances are you will need a new timing sprocket, crank pulley, and woodruff key. Hopefully the crank is not worn too badly.. but if it is, you can get your hands on some Loctite 660 Stuff is a little pricey.. I think I paid $60 for a tube.
You can get some used parts on the forums for a reasonable price, check with SV&HOdan. He has a few parts cars in his possession, and hes awesome to deal with. If not, also check out the WTB section and post an ad.
As for to fix or not to fix, it all depends on what else is wrong with the car. If it were me I would fix it. But if the car has a ton of other problems, it might not be very cost effective to fix it. You should get a price on parts for the 60k and to repair the crank, pulley and sprocket. You sound like a competent guy that knows what he's doing; so likely you will only have to pay for parts and not labor.
If the price of repair outweighs the overall value of the car to you, it might be time to move on.
Hope everything works out for you!
Shaun
How does the crank pulley look?
chances are you will need a new timing sprocket, crank pulley, and woodruff key. Hopefully the crank is not worn too badly.. but if it is, you can get your hands on some Loctite 660 Stuff is a little pricey.. I think I paid $60 for a tube.
You can get some used parts on the forums for a reasonable price, check with SV&HOdan. He has a few parts cars in his possession, and hes awesome to deal with. If not, also check out the WTB section and post an ad.
As for to fix or not to fix, it all depends on what else is wrong with the car. If it were me I would fix it. But if the car has a ton of other problems, it might not be very cost effective to fix it. You should get a price on parts for the 60k and to repair the crank, pulley and sprocket. You sound like a competent guy that knows what he's doing; so likely you will only have to pay for parts and not labor.
If the price of repair outweighs the overall value of the car to you, it might be time to move on.
Hope everything works out for you!
Shaun
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