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intermittant stumble


89blaze's Avatar
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04-29-03, 02:51 PM   #1  
intermittant stumble

I have an 89 blazer with a TBI 350. I have been having problems for months now. If you remember, I was asking lots of questions a while back. I have intermittant stumbling and every once in a while it leaves me a code to chase. I first chased a code 54 (fuel pump circuit) and ended up changing the relay, which I found bad. Then went on to a code 42 (ignition timing) which I could not find a problem with. Now I am back to a code 54 again. I got the All Data subscription to help me check all wiring and see which components work together hoping to figure this out. I have checked all electrical while in the driveway and as many as I could on the road with my multimeter, checked the fuel presure, swapped out the ECM to rule that out. Most times it stumbles for just a second then goes on fine. If the check eng light stays on, it does so when engine is running fine. Two questions for you guys and girls; will a fuel pressure check gauge fall off quick enough to show a pump cutting out at 9-13 psi (as mine should be) and what would you go with next? Am I down to a $65 an hour shop with a scan tool or does anyone think I can win this battle.

 
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04-29-03, 04:29 PM   #2  
JSStarkey1
I had a 79 Cutlass that did this. Now it was not stock & I had tossed the computer over my shoulder but after weeks of looking I stumbled on to the ignition module inside the cap. I was always told that the IM either worked or it did'nt, but I replaced it & the car quit stumbling...food for thought

 
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04-29-03, 05:18 PM   #3  
Code 42 can be a bad ignition module and or ecm,but you tried an ecm.As for the fuel pump circuit code I would do volt drop testing on the power and ground sides and look and see if I had more than 200 millivolts on either side.If I did I would divide the affected side into smaller sections to find the excessive resistance.Im not saying this is the cause of your problem but testing can save replacing good parts.

 
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04-30-03, 06:23 AM   #4  
Joe_F
I agree.

 
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05-03-03, 08:50 PM   #5  
OK, I want to make sure I am doing this right. I have always just listened for the beep(continuity) on my multimeter. What setting, 2k or 2M in the ohm settings do I use for the drop testing?

About the fuel pressure. How long does the pump need to be off to make the gauge drop while driving to see if the pump is shutting down? How long will the engine run after the pump shuts off?

Last question for now. How or why does a relay go bad? I had the fuel pump relay go when this first started. I found it by switching it with the downshift relay and after driving a while, it wouldn't downshift.

 
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05-04-03, 05:41 AM   #6  
Joe_F
Relays like any other switch or electrical device, wear out. The contacts fuse together from age, they refuse to move and close from wear, they get physically damaged, etc.

What were your fuel pressure gauge results? How do they compare to autolibrary/your service manual/the instructions with the gauge?

 
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05-04-03, 10:22 AM   #7  
I used my vacuum/fuel pressure gauge which goes up to 10psi. I need 9-13psi to be in spec. From key on, throughout the whole drive, to key off, the pressure pegged at 10. It took around 2 seconds for the pressure to fall off after key off. Since I was trying to determine if the pump was falling off, I didn't see the need to buy a gauge which would read higher.

Anyone help out with the electrical test stuff?

 
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05-04-03, 03:16 PM   #8  
Joe_F
Ummm, a gauge like that won't tell you much, it's for a carbureted car .

Get the proper fuel gauge for rent for free at Autozone. That is for FI vehicles like yours.

What service manual are you using?

 
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05-04-03, 04:39 PM   #9  
I am using All Data. I live up in the northwest where we don't have Auto Zone. A cryin shame, if you ask me.

 
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05-04-03, 06:52 PM   #10  
I took a look at the Alldata info and I see by your comments that the truck "stumbles" when this happens? Does this happen every time and then it sets the code 54? the ECM is seeing a voltage drop below 2 volts for 1.5 seconds. since you have an oil pressure switch for backup if the relay goes bad than I almost think your problem might be the 12volt feed to the relay and the oil pressure switch. Let me do a little more research.

 
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05-04-03, 07:34 PM   #11  
The truck stumbles for just a second and then goes ok. The code 54 does not light when it stumbles 90% of the time. The check eng light comes on when there is no stumble most times. Take the last two days. I leave my house, drive 1/4 mile and get on the freeway at 70mph. Drive 10 miles up a couple steep hills, get off and continue on hwy at 55. After 1 mile at 55, it stumbled with no check eng light. 5 miles later, the light comes on hard which comes up 54. I did this route both days for testing. Truck stumbled in same spot, within 1/4 mile. Second day I had the stumble with no light or code for rest of day.
I already tried the bottle of "DEMON BE GONE" I got at the witch doctors.

 
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05-06-03, 06:50 AM   #12  
Joe_F
The correct gauge isn't that expensive. Look around on the Internet or EBay or even go to Sears. I'm pretty sure Craftsman may have the right one, but it might be a special order.

 
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05-06-03, 08:25 PM   #13  
Well. looked at 2 different wiring diagrams. While driving the ECM would have to lose a 12volt signal from both the relay and the oil pressure switch. The only way this could happen is for both to go bad at the same time(not likely)or you are losing the 12 volt feed to both, due to a bad connection. If you lost this same voltage while trying to start the engine, you would get a "no-start" as niether the relay contacts or the pressure switch would have power. The 2 wiring diagrams are a little different as to the source of the 12 volts so I'll give you what I see....Alldata says that voltage comes through Fuse ECM B if your truck model is(R,V,G,P) and then it says that if your truck is model (C,K) it comes through a fusible link but does not show the origin of the fusible link. Chilton's merely states that the voltage comes from Fuse ECM B. Do you have a fuse in the fuse box(ECM B)? After the fuse link/fuse the source of power appears to be coming to the oil pressure switch and relay contact through a Orange wire and to terminal A on the relay. I would trace this wire back to the power source. Also on alot of GM trucks there is a power distribution terminal block on the engine side firewall if I remember correctly. It may be black with 1 or 2 studs and have a number of wires going to it. Pull this apart and check its condition and clean all the wire terminals. Sounds like to me you have a real bugger of an intermittent wiring problem, you might try to correlate the code 42 and see if this circuit has anything related to the code 54. Check the wiring and connector at the ignition switch also for signs of heat fatigue. Just a thought.

 
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06-09-03, 06:16 PM   #14  
Fixed

Just thought I would let you know all your help didn't go unused or unappreciated. I had checked EVERYTHING. I decided that if I pretended there was no problem, that it would go away. Kinda like when I pretended like I didn't have that anoying cat. I figured I would also take care of a few things like motor mounts and that bearing going out in the alternator. After changing the alternator, all my intermintant problems have gone away... Guess I should have taken care of that squeal a year ago when it started...

 
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06-10-03, 09:35 AM   #15  
Joe_F
Lol. Well there you have it. Might have been cutting out the power intermittently .

 
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06-10-03, 11:55 AM   #16  
89blaze, thanks for getting back to us. I guess that explains the low voltage code for the fuel pump circuit.

 
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