Falls on its face more than half throttle - TCCoA Forums

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post #1 of 16 (permalink) Old 09-04-2016, 06:41 PM Thread Starter
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Falls on its face more than half throttle

I have been chasing this problem for a year or so now, I would imagine. During the first hour or so of driving the car, it will fall on its face when I try to go WOT or anything more than half throttle. I decided my old walbro 255 was getting weak, so I installed one of those and a new fuel filter. Same problem. I think I have finally got the problem nailed down to the MAF circuit. When it falls on its face, the MAF values wont rise much over 2.5v.

I was suspicious of a bad MAF, so I bought a used one and the symptoms remain. Neither of these MAFs are dirty. I load-tested the wires from the PCM to the MAF plug with a 3157 bulb using both filaments. Result: Bright light on either circuit. The power circuit (red wire) has rock solid power as well. I am suspicious there is something going on inside the PCM. I just replaced this with a CARDONE pcm a year or two ago. Has anyone ever had a PCM go bad and cause these symptoms? All this stuff started happening after I let the car sit under my car port for like 6-9 months and the battery went dead. If anyone has any insights, let me know. Thanks

-Rob
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post #2 of 16 (permalink) Old 09-04-2016, 10:06 PM
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So after the car runs an hour, it goes "normal" or do things still act up on you?

Doesn't make sense to be a temperature related issue, as it would be at normal operating temp within 15 minutes or less.

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post #3 of 16 (permalink) Old 09-04-2016, 10:22 PM Thread Starter
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Sorry, I should've mentioned this earlier. Yeah it turns back into its old self after about an hour or so of running. What's weird is there are no codes, likely because all data values look normal except for the maf value being low. I was wondering if the ecm is warming up and "repairing" itself temporarily.

Maybe I should just grab a couple spare used pcms and flash one with the xcal. Doesn't this use one of the marriages each time it's done?
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post #4 of 16 (permalink) Old 09-04-2016, 11:22 PM
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Quote:
Originally Posted by Rob View Post
Sorry, I should've mentioned this earlier. Yeah it turns back into its old self after about an hour or so of running. What's weird is there are no codes, likely because all data values look normal except for the maf value being low. I was wondering if the ecm is warming up and "repairing" itself temporarily.

Maybe I should just grab a couple spare used pcms and flash one with the xcal. Doesn't this use one of the marriages each time it's done?
Seems like a good idea.

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PI Swapped NPI motor with some port work on the heads, PI cams, 80mm MAF, CAI that feeds from fenderwell. Jmod, 3.73:1 TL in Mark VIII carrier, Mark VIII aluminum LCAs, 93 Mark VIII driveshaft, PBR brakes (soon to be Cobras), 18x9 wheels, 255/40/ZR18 Tires, Front and Rear strut/shock bracing, GR-2 shocks, Eibach 1.5" springs, 1989 SC front and rear sway bars.
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post #5 of 16 (permalink) Old 09-05-2016, 10:49 AM
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when it falls flat on it's face, do you mean it still accelerates, but just very slowly? Or does it not accelerate at all beyond a certain RPM? If it still does accelerate, can you get the engine to the RPM redline to have the car shift on it's own up to the next gear?
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post #6 of 16 (permalink) Old 09-05-2016, 01:20 PM Thread Starter
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When wot with low maf values, the car will pop rapidly (could be back through intake, but maybe exhaust). Rpm essentially stops climbing. If I hold it for a long time it will eventually shift, but I usually let off and it shifts immediately.
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post #7 of 16 (permalink) Old 09-05-2016, 01:56 PM
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It's a different cause but the effect sounds a lot like what I went through immediately after my 5 speed swap and tune, where for whatever reason my datalogs after idle showed 6(stabilized low load) for the spark source, and showing a max advance of 15* on the log as well. Same issues, no power at wide open throttle events, and accompanied by popping and backfiringit when I tried, it was horrible.

In my case I used advantage to tune it, which it was all good with the auto, and never touched any other tables besides shutting trans controls off in the swap process. But I went back in trying to find a table filled with with 15s and low and behold there's this mysterious "max spark at low load" table, which I promptly changed to 60*(so it would never enter) and the problem was totally solved. What I never did figure out however was WHY? Could the lack of MLPS input trump actual load values to keep it in that table? something else? I suspect the answer to that question would explain both of our issues.

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Last edited by XR7-4.6; 09-05-2016 at 02:01 PM. Reason: Fixed
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post #8 of 16 (permalink) Old 09-05-2016, 05:40 PM
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Before I'd go too deep into the car, I'd probably check the air filter and the air resonator (below the air box) to see if it isn't clogged or something, it could be starving itself of air and running rich and hence the popping? Low MAF values because not enough air passing through? Maybe. I dunno.
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post #9 of 16 (permalink) Old 11-12-2016, 03:34 PM Thread Starter
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Swapped in a used mbe3 pcm the other night, car still has the same symptoms. It's getting worse too, which may help me to finally get this nailed down. Here are some things I noticed in the week or two before I put in the new pcm.

Had an extended crank last week, then when it finally started it was misfiring (running rough.) it missed and ran rough all the way to the shop, which is about 6 blocks away. By the time I got there it was running smooth, but still had the wot no power symptom.

I decided I should hook up the scanner and watch cylinder misfires (cylinder contribution test) on the next cold start. It was running rough but not counting any misfires. Still no codes. I was suspicious of he screamin demon coils I have, but did a primary resistance check and they measured the same as some other ford coils I have laying around.

After looking at some data, revving in park and getting the crazy popping sound I noticed the scanner is showing load values less than 20% and injector pulse widths of 3-4 ms, even near 4k rpm. I know the maf is used to calculate load so I unplugged it and still had the same problem. I drove it home with the maf hooked back up and Los values went near 60%, maybe because it was in gear. Anyone have any ideas? I think I just need to spend some time alone with the car.
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post #10 of 16 (permalink) Old 11-15-2016, 03:59 PM
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Check and see if it's advancing properly; the last time I had a problem like this, the centrifugal weights in my distributor weren't moving.

It's been awhile, lol.

I've had a bad mbe3 and FTE1, but never an mbe2, so far. I'm currently running 3 of them.

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post #11 of 16 (permalink) Old 11-29-2016, 07:57 PM
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Another Item to Check

I've seen this exact issue on other EFI cars before. Hard starting and running really poorly when started, but running fine after warm up. While the earlier post mentioned that the car should have come up to operating temperature much quicker than 1 hour, is usually true, there could be reasons why your engine is getting up to temp like it should.

When the coolant temp sensor for the PCM is not working, the PCM may think the engine is warm and not enriching the fuel mix to help a cold engine. This would explain why the car is running poorly and popping and also a cause of the long cranking you experienced. When the engine is up to temp, the mix is correct as the sensor is always showing as 'warm.' I am not sure if the 4.6 has a separate sender for the engine and for the fans, but thats the one to check/replace. You may able to get a reading from the PCM of the temp that the PCM is seeing, but I am not sure.

Now normally a car warms up faster than 1 hour, but a secondary condition like a stuck thermostat could cause it to take longer. Its just not a good enough 'if' to skip the checking of the sensor which will absolutely cause this type of issue.

Hope this helps.
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post #12 of 16 (permalink) Old 12-06-2016, 08:16 AM
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I had a tps go bad that was one of my 95 problem the other was those little relay things next to the coil pack I replaced those. when I replaced those the falling or bogging disappeared. but I also did a tune up at the same time replaced the plugs wires and fuel filter to knock out any other issue. that may pop up when I was narrowing it down. when I did the work to it it seemed like it needed some love. but when I replaced the tps I also replaced some of the rubber hose that went from the plentum to the break booster as well because I heard a air leak. but I would go after the tps first to see what it will do first.
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post #13 of 16 (permalink) Old 12-08-2016, 03:40 PM
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I had to replace that whats in the pic also the fuel filter as well as the fuel pressure regulator. when I ran the codes it showed three spark and a lack of pressure of fuel. so I did all four because when I did the fuel pressure regulator and fuel filter and the spark thing that I replaced it came up with a bad tps after I did all three
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post #14 of 16 (permalink) Old 12-09-2016, 05:42 PM
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I will have to agree with mach1 if the temp sensor is not ready properly the pcm will not go into closed loop. The car will be stuck in open.
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post #15 of 16 (permalink) Old 12-14-2016, 07:30 AM Thread Starter
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Ect sensor reading is fine. 95mercxr7's post mentions a part I have rarely thought of. I also have never seen one go bad on a car with dis. I'm sure this is like an ignition condenser. I assume he is talking about the little part with grey connector hooked into the coil power wires to reduce emi. I seem to remember reading about these in the shm book years ago. He mentioned always removing those since all they do is cause problems. Perhaps one or both of mine is giving up the ghost. I'm going to unhook them soon and try it out! Thanks for the good replies!
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post #16 of 16 (permalink) Old 02-25-2017, 09:18 PM Thread Starter
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Well, it's fixed. I took out my ngk plugs that melted away in a surprisingly low amount of miles. Swapped in a set of 22c plugs, a pair of new motorcraft coils and it runs awesome. I am getting rid of these screaming demon coils. So proud to have her back to her old self. Just need to finally do the gears.
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