1990 lsc that backfire pops threw the intake

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mk790lsc

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Names Rich, I have a 90 lsc with 60,000 miles. IM a mechanic but can't seem to find out what's wrong with my car. For some reason my car pops or backfires threw the intake when accelerating, plus the car hesitates. I did i complete tune-up with all ford replacement parts, I changed the idle bypass valve ,cleaned and replaced gaskets on the upper intake and throttle body. i cleaned and checked the egr and replaced the gasket. and cleaned the egr spacer plate. i replaced the vacuum hose going from the map sensor to the intake and some others that didn't look so good. the car idles great.i checked fuel pressure witch in normal. it seems to pop more when the cars cold. when i changed the plugs it looked like the cars running really lean. there no engine codes. i ran all the test from the OTC 5000 scanner that i used to check for codes. IM at a loss with it can you help.


ps the car has msd 6a plus the msd blaster coil.
but i unhooked the msd box an it still backfires.
Rich 90 lsc
 
Did you check the timing and pull a compression test? Fuel filter is not plugged, is it?
 
I changed the fuel fiter with the tune up i did an yes is on the right way. i have not checked the timing yet, waiting for snapon to get mine back.the strobe went bad.
 
Along with the other good ideas, it might be a restriction in the exhaust; something like a clogged catalytic converter.
 
I wish i could say it is the cats but there not in the car no more . This car is driving me nuts. i have had 4 1990 mark vii lsc's and this is the only one that i cant figure out. ive done some nice custom work to it too. wish i had some pics to show you guys.
 
Im hoping someone that reads this can figure this out anything you like to say please write in. im at a lose ,if you can think of things i have not tried please tell me. thankyou
 
mk790lsc said:
Names Rich, I have a 90 lsc with 60,000 miles. IM a mechanic but can't seem to find out what's wrong with my car. For some reason my car pops or backfires threw the intake when accelerating, plus the car hesitates. I did i complete tune-up with all ford replacement parts, I changed the idle bypass valve ,cleaned and replaced gaskets on the upper intake and throttle body. i cleaned and checked the egr and replaced the gasket. and cleaned the egr spacer plate. i replaced the vacuum hose going from the map sensor to the intake and some others that didn't look so good. the car idles great.i checked fuel pressure witch in normal. it seems to pop more when the cars cold. when i changed the plugs it looked like the cars running really lean. there no engine codes. i ran all the test from the OTC 5000 scanner that i used to check for codes. IM at a loss with it can you help.


ps the car has msd 6a plus the msd blaster coil.
but i unhooked the msd box an it still backfires.
Rich 90 lsc
Click to expand...
Welcome Rich.
I'm not an Auto Mechanic but I am a Maintenance Man. I've found that when 22 years of experience fails, ask the new guy what he thinks.

I'll be the new guy for ya.

I understand a backfire to be the cause of pre ignition or dieseling or a valve being open during ignition causing the explosion to be blown either out of the exhaust or into the intake.

Could you have a sticking valve or lifter?

BTW, there's a ton of members in the Queens/Nassau/Suffolk area.
 
OldSchool1 said:
Welcome Rich.
I'm not an Auto Mechanic but I am a Maintenance Man. I've found that when 22 years of experience fails, ask the new guy what he thinks.

I'll be the new guy for ya.

I understand a backfire to be the cause of pre ignition or dieseling or a valve being open during ignition causing the explosion to be blown either out of the exhaust or into the intake.

Could you have a sticking valve or lifter?

BTW, there's a ton of members in the Queens/Nassau/Suffolk area.[/QUOt\



thank you for your advise,and i was not aware that there this many people into these cars around here. i checked the timing today,it was off by alittle but i reset the timing and im still having the problem, i also did a compression test to each cylinder an that came back good. if i remember i had about 90 in each.
Click to expand...
 
thankyou for you advise and i was not aware there where that many people into these cars. i check the timing was normal. plus i did i compression test of the cylinders an thats was normal,i think i had about 90 in each cylinder. with all the people that around nass suff and queens do you guys meet up at all.
 
Just curious, did the popping happen before or after you replaced wires? Just wondering if one got crossed by accident while putting them back on.
 
no it was before the wires. it happened just one day out of no where. i was on vaction and the car was parked the whole time. when i came back it was popping. i checked the plugs and made sure they where right. i know if the 2 back wires on the drivers side are switched the car with run but back fire.
 
As an update to my car. Its still not fixed. Now i replaced the temp sensor aircharge temp sensor tps sensor map sensor and no luck the car still pops threw the intake help
 
reading back over this, the one thing that I notice is the compression. 90 PSI is kind of low. I'm not sure what the manual says is "normal" but usually I'm used to seeing 125+. If it's even all across, there probably isn't anything wrong internally other than maybe worn rings. If its low on a single cylender, I'd suspect perhaps a burnt valve.
If vacuum leaks have been ruled out, and the timing is OK, perhaps the coil is breaking down or the ignition module is going bad. Also see if the distributor shaft is sloppy. I've seen where the bushings in the dizzy are shot causing excess play and making the timing jump around at speed causing backfires and other odd stuff.
I'd also stick a vacuum guage on it to see what you get. You want to see a steady reading around 19". Higher is better, but if it goes much lower or fluctuates that indicates a problem. Steady low vacuum points to a leak somewhere. Jittering indicates a miss or a sticking valve.
 
mk790lsc said:
As an update to my car. Its still not fixed. Now i replaced the temp sensor aircharge temp sensor tps sensor map sensor and no luck the car still pops threw the intake help
Click to expand...
I'm curious Bro. Have you made any progress? I sincerely hope so because my 1990 may be next.
 
Did you ever get any resolution here ???
There are a whole lot of things that can create this condition
and would have to eliminate one at at time --
Walt
 
still no good

still have not found the problem, i had to lay up the car because of it, the car runs fine but pops threw the intake. im spend dont no what else to do. Rich
 
my mustang does the same thing

my mustang is doing the same thing after i changed my upper intake gasket i not sure if it is the gasket leaking but i am sure it was one of the vacum hoses because they looked very brittle my car only does this some time my timing is set at 13deg. never had this problem until i pulled intake usually pops around 1400-1700rpm then power comes on strong when it is acting up
 
mine was doing that

i went through alot .... then finally got a fuel pressure guage and checked the pressure up near the rails and had 15-20 pounds .....was supposed to be 35-40 min

i had a pinched fuel line between the gas tank and the under body....which i found as i was putting in the new fuel pump assy $$$

now its back to full running order :)

just another possibility....got my guage from sears for 35.00

hope ya find it
 
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