QuestionQUESTION: Hi Howard,
It's Marie, I am the one that asked you about the misfire and that it bogs out at acceleration. Replaced thermo-time switch, coolant temp switch, coil, starter relay, fuel pump, all cracked wired I could see, battery (Good Battery) and oxygen sensor. Thank you for your help, here goes.
I did the timing light test: Light blacked out at misfire while driving simultaneously, then all other time driving the light was speraticly blinking. So that says a fire problem yes? Had just replaced the coil, ran well for about 10 mins. then started to get bad changed out the Amplifier helped car run a better, but then got worst again. Recheck the coil resistance and it's at 6.53 ohm (at 200k setting.) What is ideal for the reading on the coil for my car? How do you retest the Amplifier?? Read that the pick up coil can sometimes fry the Amplifier if not changed at the same time. Is this true? The reading on the pick up coil is 3.9 ohm (at 200k setting.) (Is the correct reading for my pick up coil?) If it's not the pick up coil is there any other reason why it would have ran well then got bad again within a day?? Fuel pressure at 27 with vacuum applied and 36 without vacuum applied. Compression is at 150 Ibs. per cylinder with no variation. Haven't run vacuum test yet, total baffled about Amplifier, coil situation. FYI... I have access to running car just like mine so I know that the coil and Amplifier were in a working car. I've been swapping a few parts so that I knew the parts worked and weren't a NEW-BAD part situation. Please help, I am soooo baffled now and as I said I'm a pretty good mechanic LOL. Now I might have to buy my friend a new coil and Amplifier:( Thank you so very much! Can't wait to hear back:)
PS I might have also mentioned that I have a new head gasket, all new valves and cams.
Marie
ANSWER: Hi Marie,
The timing light test is a sure test that you are loosing ignition. Even though your fuel pressure is not correct. With vacuum you should see 32 PSI and remove vacuum and it should go to about 42 PSI but that probably would only give you stumbling but not a "cut off" and you didn't tell me if the fuel pressure tests were conducted under the failed mode conditions (running on the road).
I would confirm the ignition loss test and follow that trail. The timing light test only shows that there is a loss of output from the coil so the problem has to be the coil or before the coil (amplifier, pick-up or wiring)
Run the timing light test one more time for a conformation. Since you have changed the amp and the coil from a known good running car I believe you can scratch them off the list of possible causes. There is another test you need to run on the pick-up and that is to make the connection good to your ohm meter and then wiggle and push and pull the wires from the plug and the pick-up itself because I have found several that the coil in the pick-up (that is all it is "a coil")was good but the wires to the pick-up were broken inside the insulation. Only found by wiggling the wires with an ohm meter attached. Your ohm test was ok static but needs to be done while wiggling and pulling on the wires. The wires hanging outside the distributor are subject to an extreme amount of movement and so are the wires at the pick-up because of the movement of the vacuum advance plate. I found several by a rough test. This I did by just letting the engine idle and wiggling the pick-up wires outside of the distributor. Then to check the wires inside while running I removed the vacuum hose and applied a "hand pump "Mitty Vac" to the advance and pumped the vacuum advance up and down while it was running and found several that way. but missed one once and after a lot of chasing my tail went back and removed the cap and rotor and wiggled the wires by hand with an ohm meter attach to the leads and found a bad wire problem.
Your ohm meter reading of the pick-up is good (factory says 2.2 to 4.8 ohms) I can't say that a pick-up can't harm an amp but have replaced many of both but never have never found both bad at the same time. Keep in mind that the amp is nothing more than a GM amp. Look inside the box and you will see a GM amp available at any auto parts store. In the dealership we had to replace the box but when I was not in a dealership I have just replaced the GM unit inside.
One other possible is the alternator. An excessive charge rate can feed into the system and disrupt the amplifier. I did run into one that the diode pack in the alternator was failing and was keeping the battery up when running but was feeding a small amount of AC current into the system and the amplifier noted it and that scrambled the firing of the coil.
I only found it by loosening the alternator fan belt so that it could not charge much and a quick road test noted the ignition miss was gone. I tightened the belt and it came back and I put the alternator circuit on an a oscilloscope and I could see the spikes of AC in the pattern. So I knew to just replace the alternator.
If all this fails to find the problem you need to find a shop that has a scope and have them look at the primary section of the ignition system.
Let me know, good luck,
Howard
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QUESTION: Hey Howard,
It's Marie, the one with the 85 jag xj6 that boggs out when you accelerate and you also get a misfire. It's like you said it just mushes out and just won't hardly go and putt putt jerk jerk ect..LOL So I did the alternator test and what do you know the alternator was bad. I believe it was leaking Dc current into the system and scrabbling the coil and maybe the other parts too. I replaced it with a really good one (Bosch.) So... so far that's the Alternator, coil, amp, coolant temp sensor,thermo time sensor, battery, starter relay all bad wires in that area and of course all the work I did before...(the cams, head, valves ect...) So.....NOTHING....The car just won't run right (mostly under a load) and I feel like it just a little something, but what? Do you think it could be the ECU? I'm getting one from my friends car and I'm going to swap it and see...From my experience there are only so many things that could cause this problem so if I were to have it scoped they would only be able to tell me if there's a problem in the primary ignition system and it's all brand new pretty much. I've put too much $$$ in this car to give up now. So.. what's next. I read in the Jag shop manual that my year 85 timing should be set at 17 degrees btdc, but another mechanic told me 10 degrees btdc, I've tired both no real difference. Which one is correct? And then I thought I should check to see if my fuel is 100psi at the inlet, is that correct as well? You've been so helpful, I hope we can figure this out:) Thank you so very much for your time and help, can't wait to hear back from you.
Thanks, Marie
AnswerHi Marie,
The fuel pressure should be measured from a "T" put in the fuel line just before the fuel rail and it should be about 32 PSI with vacuum on the regulator and about 42 PSI with the vacuum removed.
Did you try removing both hex plugs on the exhaust manifolds? to see if it was a partial stopped up exhaust system. Or just remove one and put a low pressure gauge in and rev the engine up to see how high the pressure goes. The engine can not handle more than 1.5 PSI when reved up or put it in gear with someone holding the brake to see if it will go higher under load. A stopped up CAT or muffler will show much higher readings. Another symptom of a partial stopped up exhaust is manifold vacuum dropping below 5 in under full throttle and full load.
17 deg will probably give you the best fuel mileage and power but the idle (which is not great anyway will also suffer) Some Series III cars have a hard time passing emission tests with 17 deg timing. 10 deg will get you a better emission test and a little better idle but the mileage and power may suffer.
let me know, Howard
You seem to be much sharper than most people on tech so you may find my tech tips on my web site of some use. If you get time, take a look. http://mg-tri-jag.net
