Missfire/bog (Page 1/4) XxdjxX89 MAY 28, 12:25 AM This issue I have has got me baffled and as a mechanic it’s been ive been fighting this for 2 months , let me start from the beginning. I was driving my 86gt with 2.8 to work and on my way under light throttle the car would have a slight hesitation , first thing in my wind was let’s do a tune up so I went to replace the cap and noticed the distributor had corosion and rust , I’m better off replacing the distributor , so I did , well that didn’t solve the slight hesitation so I dug farther in,, I did spark plugs and wires of course but I noticed they were arching on the head 🤔 I ended up trying the old distributor and they still arched on the head , the plugs had no sign of carbon tracking , so I replaced the wires with good set of Taylor wires hoping that would solve it and nope still arch on the head ,, ended up putting new dist back in , clean/ fixed every ground possible , swapped ignition coil , still had arching on the head , ditched Taylor wires and put on msd wires , arching on the head is now gone but the spark plugs glow blue at night ,,, I decided maybe let’s move on to fuel ,,, I had my injectors flow tested , fuel pressures holds with key on ,,, when hesitation happens there is no drop in the fuel so I still believe it’s something else ,,, checked for vacuum leaks can’t find any , throttle body and all parts are clean ,,, did timing chain to rule out possibility of engine timing being off ,,, ignition timing is set at 10 degrees btdc ,,, I’m so lost compression is even all cylinders are around 120psi I tried swapping sensors from known working fiero’s , we have 4 of them , iat,map,tps ,02,iacv, CT’s, Car still has hesitation , if anything it’s gotten worse , I dropped fuel tank and is amazingly clean replaced fuel pump , just to rule that out , fuel preasure regulator works the way it should , tried another ecu that is from say year / trans and made no difference,,, if anyone has any guidance or anything they can recommend what I might be missing let me know ,,, I even tried unplugging alternator but made no difference, I had a classic car ware alternator had bad diode and would cause a missfire , please anyone just get me in the right direction Patrick MAY 28, 01:49 AM Have you checked to make sure the EGR valve is actually closed when it's supposed to be? XxdjxX89 MAY 28, 10:47 AM Yep egr functions as it should , diaphram is also good This hesitation, happens in neutral and under load XxdjxX89 MAY 28, 05:31 PM Back at it again , I decided to replace my flow tested injectors with brand new accel 15lbs and sure enough made no difference, I think it’s safe to say it’s not fuel injectors ,,, I have 42psi of fuel with key on car off , 36psi with car running took it for a drive and fuel doesn’t drop below 34psi under load so pretty sure that’s good , car off and fuel doesn’t bleed down , holds perfect! Safe to say what ever is going on its not fuel related Rechecked gap on spark plugs using r42ts plugs , all are at .045 Ignition timing still at 10degrees btdc Tried another known good map sensor made no difference Car idles great ,,, whe miss/ hesitation happens when throttle is applied Air filter is new , no blockage in air system ,,, tried another tps , no difference 🤦‍♂️ I’m baffled at what ever is causing this hesitation, bog I guess better word for it ,,, cat is clean , no exhaust restrictions / no exhaust leaks ,,, Patrick MAY 28, 05:58 PM quote Originally posted by XxdjxX89: Ignition timing still at 10degrees btdc And you had the ALDL jumper in place when you did the timing? XxdjxX89 MAY 28, 06:10 PM Yes sir Patrick MAY 28, 06:50 PM Next thing I'd suggest checking is to see if the outer ring of the harmonic damper might've slipped. If it has, then the timing marks are no longer correct. This is much more of a problem than some people might suspect, especially keeping in mind that the original units can be 35 years old. With something like a paper straw inserted through the spark plug hole, determine TDC of cylinder #1 and see if the wider of the three timing grooves lines up with 0° on the scale. XxdjxX89 MAY 28, 07:09 PM Timing mark is good , one thing I forgot to mention is this car did end up with a slipped harmonic balancer and I bought one from the Fierostore , I matched it up to the old one and added the mark to the new pully where it should be , but for safe measures I’ll double check the height of piston just to rule that out hunter29 MAY 28, 07:45 PM Arching on the head ? Head ? Cap ? XxdjxX89 MAY 28, 11:15 PM No arching on cap or coil was only at cylinder head , , I want to thank you Patrick for the help I may of just found the issue , the tdc mark on the new pulley that was marked is not at true tdc of the motor , I’m going to redo valve lash and post back what my status is