Engine misfiring
02-05-2011, 08:06 PM
#1
Engine misfiring Got a 94 3.8 liter. Compression is good (180 psi all cylinders). Vacuum looks good, reading fairly steady 18in, but when it starts missing it will drop and come back. Spark appears good. Plug wires have no more than 2500 ohms resistance per foot. Each one fires when a spark tester is used. Spark plugs have less than 20,000 miles on them (bosch platinums).
I thought it was the fuel pump as I only read 34 psi when the car was running, would jump to 44 psi when the vacuum line to the regulator was removed, which was good. Spec for the fuel pump is 41-47 psi, but thats when the key is first turned on. I get 40 psi, which I figure 1 psi low is probably more related to accuracy than issue.
So what the heck is causing my misfire? The issue seems to get worse when it gets warmed up. It is most noticable when sitting at idle in gear or when accelerating.
I thought it was the fuel pump as I only read 34 psi when the car was running, would jump to 44 psi when the vacuum line to the regulator was removed, which was good. Spec for the fuel pump is 41-47 psi, but thats when the key is first turned on. I get 40 psi, which I figure 1 psi low is probably more related to accuracy than issue.
So what the heck is causing my misfire? The issue seems to get worse when it gets warmed up. It is most noticable when sitting at idle in gear or when accelerating.
02-05-2011, 09:18 PM
#2
On The Tree
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how does the maf look tried cleaning it? could be alotta things do you have acsess to a scope? mabey see how the ckp signal looks like you could pull a pattern on your injectors also ohm test thoose there is usually a spec on injectors.
02-06-2011, 06:39 AM
#4
Codes?
how does the maf look tried cleaning it? could be alotta things do you have acsess to a scope? mabey see how the ckp signal looks like you could pull a pattern on your injectors also ohm test thoose there is usually a spec on injectors.
I was thinking it was related to the egr, but it is the digital model on this one and without a tech 1, I cannot do a test on it. Then again, I dont think its quite in closed loop when the issue starts either. It get noticable about a minute after the car starts running.
02-06-2011, 08:20 AM
#5
went out this morning and ohmed the injectors, they are all with in spec. I read that if you unplug the MAF and the issue went away, then the MAF is the issue. I thought I'd warm the car up first so I went and drove it to get it up to operating temp and to my surprise, it did not have any issues.
So, I thought to myself, what is different here. I noticed that when I turned on my headlights, it felt like it missed again. the volt meter dropped from 14-13 and went back up and ran normal again. I sat in drive with my foot on the brake and on occasion it would drop miss and the volt meter would drop and come back. Could this all be related to ground? I question this because I replaced the fuel pump (as I thought it was the issue) but it did not fix the issue. I checked voltage at the connector to the pump and it read 8.6 volts. So, i thought the wiring to the pump was the issue. I went through and checked voltage at the relay and it had only dropped .5 volts from the battery. So, I went back and check voltage at the connector again and this time it was the same as the relay. I back probed the connector with the engine running and it was still battery voltage.
So, I thought to myself, what is different here. I noticed that when I turned on my headlights, it felt like it missed again. the volt meter dropped from 14-13 and went back up and ran normal again. I sat in drive with my foot on the brake and on occasion it would drop miss and the volt meter would drop and come back. Could this all be related to ground? I question this because I replaced the fuel pump (as I thought it was the issue) but it did not fix the issue. I checked voltage at the connector to the pump and it read 8.6 volts. So, i thought the wiring to the pump was the issue. I went through and checked voltage at the relay and it had only dropped .5 volts from the battery. So, I went back and check voltage at the connector again and this time it was the same as the relay. I back probed the connector with the engine running and it was still battery voltage.
02-13-2011, 06:16 PM
#6
Engine Misfiring, need some gurus. still needing some ideas here. as i drove it last week, it consistanly got worse each day. No check engine light has come on. Went through and cleaned the grounds, they all looked good anyways. Only one I havent found is the ground to the fuel pump. Cleaned the positive terminals and the connection to the starter. Cleaned the MAF and replaced the O2 sensor. No change in how it runs.
Other items I forgot to mention. about 2 days prior to this happening, I sea foamed the thing. Not sure that it would have caused all this.
About 9 months ago, I added a remote start system to the car. It is still functional, so I wouldnt think its causing an issue. If it was causing the issue, would it be related to the pink ignition wire it is tied into?
Is it possible the computer has **** out?
Other items I forgot to mention. about 2 days prior to this happening, I sea foamed the thing. Not sure that it would have caused all this.
About 9 months ago, I added a remote start system to the car. It is still functional, so I wouldnt think its causing an issue. If it was causing the issue, would it be related to the pink ignition wire it is tied into?
Is it possible the computer has **** out?
Last edited by 71novaguy; 02-13-2011 at 07:34 PM.
