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Old 07-27-2013, 02:32 AM   #1
smurf5995
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Default 1986 engine stumbling

Need some help figuring out this problem. My 1986 4+3 has developed a stumbling problem upon acceleration at any speed. The symptoms seem to get worse the hotter the car gets. The plugs are about 3 months old and I have pulled them and they are fine. I have replaced the plug wires and everything inside the distributor along with a new cap. I replaced the MAF and the TPI. I am down to pulling the plenum and checking my injectors, but short of this, does anyone have any other suggestions as what might be causing this problem?

BTW, the car is idling fine and when I hold the speed steady, there is no hesitation...the engine seems to run GREAT other than the hesitation/stumbling upon acceleration.
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Old 07-27-2013, 03:16 AM   #2
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Have you checked the fuel filter and pump? An old pump that's failing to send enough fuel upon demand could cause your symptoms.
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Old 07-27-2013, 07:32 AM   #3
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Have you checked the fuel filter and pump? An old pump that's failing to send enough fuel upon demand could cause your symptoms.
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Old 07-27-2013, 08:02 AM   #4
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Quote:
Originally Posted by smurf5995 View Post
Need some help figuring out this problem. My 1986 4+3 has developed a stumbling problem upon acceleration at any speed. The symptoms seem to get worse the hotter the car gets. The plugs are about 3 months old and I have pulled them and they are fine. I have replaced the plug wires and everything inside the distributor along with a new cap. I replaced the MAF and the TPI. I am down to pulling the plenum and checking my injectors, but short of this, does anyone have any other suggestions as what might be causing this problem?

BTW, the car is idling fine and when I hold the speed steady, there is no hesitation...the engine seems to run GREAT other than the hesitation/stumbling upon acceleration.
A good test to do when engine is cold, (helped me find a faulty injector electrical connector plug)
Start the car from stone cold and run for 10 to 20 seconds, then check the headers an inch from the head if one is cold you can concentrate on that cylinder. The injector plugs can be swapped so say cylinder 2 is cold swap cylinder 4 injector connectors and see if the fault transfers, the injectors all operate at the same time via the ecm.
Then check the ignition lead with a multimeter and swap spark plugs, yes new plugs can be bad... the injector connectors are readily available (never remove them by tugging at the wires !!)
The standard injectors are at the end of their life almost, also bio fuels can damage them.
Good luck
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Old 07-27-2013, 08:56 AM   #5
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Could be as simple as a worn TPS. Get a scanner on and watch for proper operation.
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Old 07-27-2013, 03:14 PM   #6
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Even though the plugs are new, pull them and check that they are all equal in color. If there are any that are too white or too dark, you need to check the corresponding cylinder. Check your injectors for resistance with an ohm meter. The resistance should be somewhere in the neighborhood of 16 ohms if memory serves me correctly. I have seen too many of the tuned port cars suffer from injector problems. We have had luck in the past by sending the injectors out to a place that sonically cleans them and replaces the filters inside and the o-rings, when there is a fuel delivery related issue. Alot of people don't know this, but the injector o-rings also work as an insulator to help cool the fuel as it travels thru the injector and before being sparyed into the combustion chamber. The cooler the fuel, the better the performance. Is the rest of your secondary ignition up to par? If you can, pull our distributor cap and check for excessive carbon tracking on the inside of the cap and on the rotor. Also check for oxidaion on the dist. cap towers (where the plug wire plug into the cap)
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Old 07-27-2013, 10:11 PM   #7
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Other things that I have done are: replaced the fuel filter, I had the fuel pressure checked by a mechanic (a real one), throttle position sensor has been changed, checked the voltage going to the injectors, I have pulled the plugs and checked the gap as well as coloration, I have replaced just about every sensor except for the EGR, ESC, and the IAT. Mind you, I am getting no OB codes.
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Old 07-27-2013, 11:00 PM   #8
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check timing??
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Old 07-28-2013, 01:34 AM   #9
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Clogged cats?

Edit: I'm so sure it's clogged cats.
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Old 08-03-2013, 11:55 PM   #10
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Nope, cat is not clogged...I did set the timing and then set it again after I noticed the computer set the harmonic balancer mark to the 12 o'clock position. Each time I timed it, the mark goes to the 12 0'clock position. I would indicate degrees, but I do not know how many degrees this is since the mark is not even registering inline with the timing tab unless the computer is unplugged.
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Old 09-01-2013, 10:22 PM   #11
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Fixed...it appears it was the EGR valve. However, note the plenum was a PITA to get back on. Very easy to remove though. Something was binding it so that it would not go back on smoothly. It took a little work and effort to align the bolt holes. I ended up starting the bolts on one side, and then worked the bolts on the other side. But hey, hey no more stumbling.
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Old 09-01-2013, 10:32 PM   #12
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MY '86 has developed a funny stumble that would seem to magically come and go. This past week it got worse. So I started checking. Yesterday I got around to testing the EGR operation and sure enough I couldn't get ANY movement out of the EGR diaphragm not matter how much vacuum I applied. Ah well, I think I replaced this about ten ago, guess it was due....
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