Rough idle/shaking issue


fritz84

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Recently purchased a 1994 Ranger w/2.3 and auto trans. The truck has ~130K miles on the original engine and a recently rebuilt A4LD trans. I just had the timing belt (and thermostat/water pump) replaced. In addition to basic tune up (new OE plugs and wires, air filter, fuel filter) and oil change type maintenance, I have been attempting to track down the source of rough idle/shaking issue that remains elusive. I have searched for and read through threads reporting similar issues and have thrown a number of parts at the truck to no avail, they include: MAF senor, TPS, coolant temp sensor, IAC valve, PCV, and upstream O2 sensor. Where relevant I tried first cleaning the items in this list before replacing, but no formal testing. They were all original parts so I figured no harm in replacing them, even though possibly not necessary. The check engine light is not on nor was I able to get any recoverable problem codes using an OBD1 scanner. This truck did not come with a EGR valve (determined based on looking for it and confirming with reference to vin). I (and a couple of mechanics) have looked for vacuum leaks and only one small one was found and addressed. There is no doubt the throttle plate adjustor screw has been monkeyed with previously, and I adjusted it again following the protocol described in one of these forums (maybe the Explorer/Ranger site), where you are advised to back it out to the point where the engine just barely remains running with the IAC unplugged. While the truck didn’t come with a tach from the factory I have since wired one up using the dedicated wires from the instrument cluster. I have read where the rpm’s should be around 750 at idle (with the IAC plugged in) once the engine is warm. I’m not sure if the aftermarket tach I have is reading true (AutoMeter 2306) but it functions and provides some frame of reference. So, after adjusting the throttle plate screw and plugging in the IAC the rpm’s definitely jump up as expected and suggesting it is working as it should, at least more or less. On a cold start the rpm’s jump up to maybe 1500 initially but then falling very quickly to 1000 until the engine reaches operating temp by which time rpm’s settle at 500 and the rough idle and engine shaking/apparently low idle condition is evident. The vibrations are most noticeable in the steering wheel, but also extend to the dash. The condition is most noticeable with the transmission in drive with the vehicle stopped compared to when in park, but not dramatically so. Standing outside of the truck while it is running you don’t sense the carriage is shaking, although the sound of an irregular/low idle is evident. The engine does not threaten to stall, and these symptoms pretty much go away (or are masked) when driving. I have reset the computer following all of the major adjustments in order to let the system ‘re-learn’ the air/fuel mix information, as seems to be recommended. The engine and transmission mounts are in good condition, having all been replaced recently. Finally, I’ll just mention that the exhaust system is original, as best I can tell, and it looks like it could stand to be replaced, though is not blown out by any means. If I can’t figure this out with your help I’ll have to search for a mechanic who is more familiar with these engines etc than I have encountered so far. I appreciate any relevant suggestions/experience you might be able to share…
 


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RonD

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Welcome to TRS :)

You should use paragraphs, very hard to read one long sentence, just FYI


4cyl engines need a higher idle so they don't shake, they only fire every 180deg so by nature will vibrate at lower RPMs
So 500 RPMs is just too low for warm idle, should be 750-800 on any 4cyl engine, manual or automatic trans

The computer should set that minimum idle, if yours is not then I would says its a computer issue
The cold start REV is normal, IAC Valve is open all the way for startup
The drop to 1,000-1,100 after a few seconds is correct, computer is closing IAC valve to set cold idle based on ECT sensor's temp reading
So it reads like IAC Valve is working like it should

I would replace ECT sensor just on spec, but can't see computer going below its minimum idle setting, even if temp is reading high

You can pull out computer and open it up

Should look like this: http://www.auto-diagnostics.info/ford_eec_iv

The 3 Blue capacitors you can see in the picture can leak after 20+ years, this can cause odd issues in operation of the computer

Also test the Motor Mounts
Open hood and start engine
Look under the hood at the engine
Shift in to Reverse and watch engine shift positions, give it a little gas, then let off, if engine is lifting up then motor mount is broken
Shift in to Drive and do the same test

Engine will shift a little, but not alot or lift up
 

fritz84

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Thank you for the detailed response- I appreciate it! Probably related to the presentation of my query, but those additional items you list (ECT and mounts) to check can be crossed off as well. I was afraid it might be an issue with the brains. I guess I'll have to give that a look and operate or replace if necessary.

Does it surprise you that no codes would be thrown if that is the culprit? And this question may well become obvious once I open it up, but can those capacitors you mention be replaced if damaged?

Thanks again,
 

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Codes are strictly emissions related. If the brains are fried then they may not "know" they are fried.
Caps can be replace if you know how to solder. Its probably junk anyway, why not practice before buying a new/used one?
 

RonD

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+1 ^^^

Hate to say it but after engine is warmed up you can adjust the anti-diesel screw to open throttle a bit to set minimum warm idle of 750 or so, where ever is smooths out

Also unplug IAT(air temp) sensor, or ECT sensor and drive it a bit to see if it sets a code, basically a sanity check of computers ability to set codes
 

fritz84

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I'm more than willing to try a few more things before replacing the ECU, including soldering on the motherboard and more attempts to find the Goldilocks position the anti-diesel screw. At one point I had the screw set up where it would idle at ~750 when warm but when I then unplugged the IAC to check to see if it was participating there was no change in idle speed- implying the screw was turned in too far (note this was done with the brand new unit).

Without having an independent calibration of that tach I'm suspicious it may not be giving an accurate baseline, but only on the basis of one of the mechanics observing (based on ear and experience) that it seemed to reading low, but who knows. The subtly rough idle and shaking were present at that time. The fact that it doesn't stall and the shaking is not yet violent makes me also wonder if it can be lived with. That same mechanic, who granted had limited experience with this vintage, also said he thought it might just be the way they run- something I remain suspicious of.
 

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Sounds weird but a fuel pump did it for me!
My '94 shook like a diesel at idle for a while. Then it wouldn't start while hot, then it wouldn't start at all.
New fuel pump and she was right as rain til the ECU chewed on a pistol. Now she's getting carb'd.
There's a Schrader valve on the fuel rail near the back. Hook up a gauge. You should have 30-45 PSI at idle and 35-45 PSI in KOEO. If not, pump & filter. If you have the right pressure, back out the idle speed screw a bit on the throttle body.
 

fritz84

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Appreciate the suggestion, however can rule that possibility out because it was recently replaced too, though not listed in the original post. The gas gauge didn't work when I got the truck and while it may just have been the sending unit I dropped in a new complete assembly due to age.
 

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About how long ago was that, and did the issue persist before the fix or did it start appearing shortly after?

I'm sticking to my guns on the fuel pressure test. With the junk China is flooding the market with I wouldn't be surprised if your new pump was defective - hence service manuals calling for "known good" parts nowadays, new doesn't always mean good and good doesn't always mean new. If the issue is intermittent, make sure you are experiencing the issue when you run your test. You also have original fuel lines, yes? Ethanol had only been in use for maybe 6 years when our trucks were made. During my latest project with the truck I rolled underneath and found rubber chunking off the outside of the lines. Definitely worth looking into.
 

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The fuel pump assembly (Delphi) was done around 3-mos back, and I'd say the issue has remained approximately the same following its replacement. I'll follow up on your suggestion though, since the problem remains elusive and this is one thing not yet checked. To my knowledge the injectors have not been evaluated for issues either, though its possible one of the mechanics who worked on it checked them and just didn't mention that to me.
 

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Regarding the condition of the fuel lines, while I'm not sure if they are original or not, I believe so. They appear to be in good condition, having inspected them when I replaced the fuel filter.

To clarify, the rough idle/shaking condition has not been intermittent, rather is consistently how the engine behaves once up to temperature. The condition goes away or is masked when driving, and upon cold start until warmed, so when running at higher rpm's.

Didn't mention doing so before but I recently ran a full tank of ethanol free gas through the system and there was no noticeable difference in performance related to that.
 

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I just had a chance to pull the computer out and look it over for obvious issues, focusing on leaky capacitors. I can't see any evidence of any such issues so am going to reassemble and put back in. While this doesn't eliminate it as the source of the issue I'd like to poke around a little more before swapping it out.

I came across this youtube link searching for 'how to' related to undertaking the above type of repair which provides useful guidance if you can spot obvious issues...https://www.youtube.com/watch?v=B0Dj40Dkszo. OBD-I computer in a 92 Ford F150.

Before removing the computer I unplugged the IAC (after the engine was warmed) and there was no change in rpm's (steady at 500). This came as a surprise having set the anti-diesel screw at point where the engine was stumbling without it plugged in recently, following the reset protocol. I backed the screw out again before plugging the IAC back in and while the idle speed was maintained at a slightly higher level (600 rpm) but has since dropped back down to 500.

Any chance the ignition control module (ICM) is causing the low idle/shaking behavior I'm experiencing? Is there a means of testing that component/should it throw a code?

Thanks,
 

fritz84

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Should have rolled this into the previous message, but also wondering if there might be any benefit to getting the computer reflashed, if that is even possible (a dealer item I assume)?
 

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I apologize for the late reply. Regarding the computer, standalone models are still sold but they aren't cheap. Ford can (likely?) still reflash yours at the dealer - also not cheap. I don't know too much about the computers in these trucks (I'm actually doing a computer delete on my truck...) so I can't provide too many resources while on lunch break today.

You can also adjust the throttle cable so that your throttle plate is held open slightly more at idle rather than relying on the IAC. To do this, look at the plate your throttle cable hooks into. At the base of the plate you'll see a little "finger" with a screw touching it. Screw that in to increase throttle plate opening and screw out to decrease.

Any possibility your cat is plugged up or you have a worn lifter?
 

fritz84

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Appreciate your suggestions. I'm dubious about it being the computer, and they are kind of expensive to boot.

I have played around with the screw a little already, and that might end up being the short-term solution, although it doesn't address the source of the problem, at least as I understand that system.

I've wondered about any role that the exhaust system may be playing in this but other than for replacing the upstream/only O2 sensor haven't dug into that. As far as I can tell it is still the original system, and while rusty seems to be intact and functional. No doubt the cat could be worn out and plugged up. So, replacing the exhaust is on the list and may be the next thing I will do...
 


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