- This topic has 20 replies, 4 voices, and was last updated 9 years, 4 months ago by
.
-
Topic
-
Hi guys,
1990 Astro Van running bad; hesitating, losing power when accelerating, and stalling out. It just started this within the past few months.
Assuming this is either a fuel or electrical problem, and in order to avoid part jumping, how can I determine which it is? If it’s electrical, I’m thinking Plugs, Distributor, etc. If fuel, then either the pump (in-tank), filter, etc. What would I be missing? Could also be a vac leak, but from where? I haven’t located any unhooked hoses.
What’s Been Done:
1. A mechanic has replaced (he said) the throttle body, assuming a crack within. There was a good amount of vacuum heard before this, indicating a leak. His repair has allowed only that the truck can remain in a started state to get me home, but the hesitance is still there, and I know at any moment I will find myself embarrassingly stalled in the middle of an intersection.2. I did a voltage reading on the MAP sensor, and I got 4.8v on both the power and signal side. Since I didn’t have a vacuum pump, I tested this with the vac hose connected/disconnected, with no change in voltage. I also disconnected the vac hose and sucked on it while measuring voltage, and the voltage dropped from 4.8 to to 3.8 or so.
I’m sure I left out a lot of pertinent info, please just let me know what you need to know, and I will supply what I can.
BTW: can we upload pics here?
Thanks, and apologies for the long post.
-
Replies
-
[quote=”winchester” post=137788]1. A mechanic has replaced (he said) the throttle body, assuming a crack within. There was a good amount of vacuum heard before this, indicating a leak. His repair has allowed only that the truck can remain in a started state to get me home, but the hesitance is still there, and I know at any moment I will find myself embarrassingly stalled in the middle of an intersection.[/quote]
That doesn’t mean there still isn’t a vacuum leak somewhere. BTW, Welcome, and yes, you can post pics.2. I did a voltage reading on the MAP sensor, and I got 4.8v on both the power and signal side. Since I didn’t have a vacuum pump, I tested this with the vac hose connected/disconnected, with no change in voltage. I also disconnected the vac hose and sucked on it while measuring voltage, and the voltage dropped from 4.8 to to 3.8 or so.
That sounds normal. These MAPs should read 4.8 on the signal at normal pressure (14psi, 1bar, no vacuum). Dropping to 3.8 when vac applied is appropriate behavior, but without a vac pump where you can measure the applied vacuum you can’t be sure the voltage drop is correct for the amount of vac. If the car was running when you did the voltage test with the hose connected and you got no voltage drop, there wasn’t any vac on that hose at idle, which is not correct, and indicates a bad hose, restriction (not likely), or a vacuum leak in the system such that vac was not available. The engine produces the most vacuum at idle and deceleration, so you should be seeing vac on that sensor at idle.
For more ideas, review Eric’s videos on performance issues.
[quote=”cap269″ post=137794]
2.I did a voltage reading on the MAP sensor, and I got 4.8v on both the power and signal side. Since I didn’t have a vacuum pump, I tested this with the vac hose connected/disconnected, with no change in voltage. I also disconnected the vac hose and sucked on it while measuring voltage, and the voltage dropped from 4.8 to to 3.8 or so.
That sounds normal. These MAPs should read 4.8 on the signal at normal pressure (14psi, 1bar, no vacuum). Dropping to 3.8 when vac applied is appropriate behavior, but without a vac pump where you can measure the applied vacuum you can’t be sure the voltage drop is correct for the amount of vac. If the car was running when you did the voltage test with the hose connected and you got no voltage drop, there wasn’t any vac on that hose at idle, which is not correct, and indicates a bad hose, restriction (not likely), or a vacuum leak in the system such that vac was not available. The engine produces the most vacuum at idle and deceleration, so you should be seeing vac on that sensor at idle.[/quote]
Cap, Thanks for the reply.
I think I’m going to change the plugs and wonder if you can help me with that. This engine is a C1500 (1990). Can you recommend the correct plug for this engine? It is a V6. ALSO, Where could I find the correct gap for the plugs; I don’t have a manual.
I’m also suspecting a fuel pump problem, but I’d like to use a gauge to find the pressure but I don’t know where to check from. Do you have any exp. with this engine?
The OEM specified plug is AC Delco CR43TS. Make sure that C is in front. R43TS are same plug but with thinner electrodes. The gap spec is 0.035. I agree with Eric that only the OEM specified plug should be used. I also agree with Eric about the plug gap. I also haven’t gapped a plug in ages, since I worked on carbureted vehicles. The gap out of the box is usually correct, but it doesn’t hurt to double check.
I am assuming you have the TBI system. On the throttle body unit there is a flexible fuel line that comes from the fuel filter. You will want to disconnect this line and connect the fuel gauge between this line and the throttle body. A tee adapter is usually used in this application. Also, a pump may be able to deliver pressure but with reduced volume. There are two tests to run, the pressure test and the volume test.
Pressure test:
Test 1) static pressure: this is the pressure received from the fuel pump prime, key on engine off. The spec for this is 9-13 psi. After fitting the gauge, you may have to cycle the key on/off to remove air and let the pressure build.
Test 2) holding pressure: Turn off the key. After the pump cuts off, watch the pressure to make sure it doesn’t drop. If it falls back to zero, there is an issue with the fuel pressure regulator (or check valve in the fuel pump).
Test 3) running pressure: crank the engine and let it run at idle. the fuel pressure should remain steady between 9-13 psi.If the static pressure in test 1 never reached correct pressure but it did reach pressure in step 3, there could still have been air in the line, so re-run test 1. If it still fails to reach pressure, the fuel pressure regulator is faulty or there is a restriction in the fuel filter. Replace the filter and re-test. If the pressure remains low after re-testing, the fuel pressure regulator or pump is failing. Continue with the volume test to determine which part is failing (the volume test does not involve the regulator, so if the volume test turns out good, then the regulator is bad).
If the running pressure in step 3 was higher than spec, there is a problem with the fuel pressure regulator. If the pressure was lower than spec, or the pressure was fluctuating, there could be a problem with the fuel pressure regulator or the fuel filter. Replace the filter and re-test. If the problem remains the fuel pressure regulator is faulty.
Volume (flow) test:
Remove the fuel pressure gauge and allow the end of the fuel line from the filter to rest in a clean container. Turn the ignition key on (do not crank) for 15 seconds, then turn off. In the container, there should be 1/2 pint (8 oz) of fuel. If less than this amount was delivered, and the previous pressure test was within spec, there is a clog in the filter. If the pressure test was low or below spec, the filter has a blockage or the pump is failing. Replace the filter and re-test. If the problem remains after the re-test, the pump is failing.Well, I haven’t had a chance to investigate the fuel pressure yet, but I plan on it the next couple of days. In the meantime…
The truck is running but it is still wanting to die, both at at idle, and when I drive it. I am able to keep it running though. Does this sound like a fuel pump problem? I mean, I would think that if the pump is bad it wouldn’t start back up after all the problems I’ve had.
Obviously the pump is working or I wouldn’t be able to drive it at all.
Another symptom I thought of is when I first brought it back from the shop, it would surge a bit, but that seems to have gone away.
[quote=”winchester” post=137951]Obviously the pump is working or I wouldn’t be able to drive it at all.[/quote]
Yes, of course, that just means that the pump hasn’t completely quit.
Does this sound like a fuel pump problem? I mean, I would think that if the pump is bad it wouldn’t start back up after all the problems I’ve had.
Ok, let’s remember what the fuel pump, or any pump for that matter, does. It creates FLOW. The modern electric pump has more than one magnetic stage in it. For the sake of simplicity, let’s say that your pump has three stages. If one stage has gone bad, the other two are still working, so you are getting some flow but it is only 2/3 of what it should be. That means that after the fuel leaves the pump, passes through the lines and filter, and gets to the engine, it has pressurized, but at reduced volume. So, even though the pump hasn’t quit working, in this case, it isn’t working the way it should be. That is why if you look above at the testing method I provided it tests BOTH pressure AND volume. It isn’t enough to know that the pump is coming on and that the pressure is hitting the right number, you must also know that the flow volume is correct. The engine is going to demand a certain volume of fuel at the correct pressure in order to run properly. If the pressure is bad or the flow is reduced, the engine will starve for fuel and have performance problems.
I’ve had 2 AWD astros, overall they were great vans. They both had fuel pumps fail. I kept one long enough to have to replace the pump twice.
Spark plugs are no fun on those.
I got the pressure gauge the other day and am trying to work with it today. No luck so far. Not sure how to connect it. Here’s what I’ve got:
The flex line that you spoke of between the filter and TB is accessed from underneath the van, so I take that apart. I now have two lines (one from the filter, one going to TB).
There is what I believe to be the t-fitting you spoke of:
[URL=http://s249.photobucket.com/user/tacticaltal/media/tfit_zpslhxfvypg.jpg.html][IMG]http://i249.photobucket.com/albums/gg201/tacticaltal/tfit_zpslhxfvypg.jpg[/IMG][/URL]
This screws into the gauge, but how to connect the two ends? I have two rubber hoses, but neither is big enough to fit the metal end going to the TB, and the end going to the filter is a female, and so no hose is going there, so I don’t believe I’m supposed to be using the rubber hoses to connect the two lines. Either that, or the above isn’t the t-fitting you spoke of.
What am I missing?
EDITED TO INCLUDE: The flex line going to the TB has a metal male connector, while the line going to the filter has a female metal connector.
It won’t help for the flow check, but theres a schrader fitting on the fuel line at the back of the engine, accessed by removing the doghouse. At least there was on my late model Astr0, not positive on the TBI models. (pre-1996-ish?)
If I was doing it often, I’d go to a junkyard and grab that fuel line of a wrecked van and cut the filter fitting off.
Sorry, kbeefy, there are no schrader valves on these.
There are purpose-made test adapters for GM TBI (that you can optionally permanently install because it has a schrader valve in it) but you can fashion your own test tee with brass plumbing parts. You do not have to test under the vehicle. I usually test right where the line connects to the TBI, even though it is a hard line there. Unscrew the fuel line from the TB and attach adapter to TB where the fuel line was, connect the fuel line to the adapter, and finally connect the gauge to schrader port on the adapter. When I had my ’88 S10, I just cut the hard line a few inches below the TB with a tubing cutter and would connect my gauge using rubber hoses and clamps to the cut ends of the hard line. When test was complete, I would slip a 2-3 inch piece of fuel line over the place where I made the cut and clamp it down on each end. Then whenever I wanted to re-test, just undo the clamps, slide the little piece of hose down, and connect the gauge back up.
GM TBI fuel test adapter:
OK, I’mm a bit confused, but if I use that adapter (if it fits at the TB) I won’t need the tee, right? I will connect the end you marked TB fuel supply port to where I took the fuel line off, and the other end (fuel supply line) will go to the line from the filter.
It should have fit. Maybe try fitting it at the fuel filter? It should fit at least one of those places. You can test the way I did, if you don’t have a reservation about cutting the supply line. All you need is a tubing cutter, a piece of fuel injection hose that fits snug over the hard line, and a couple of clamps. Otherwise you will have to find some way to use standard plumbing parts to get it fitted (I believe the threads on the fuel lines are metric).
[quote=”winchester” post=138400]OK, I’mm a bit confused, but if I use that adapter (if it fits at the TB) I won’t need the tee, right? I will connect the end you marked TB fuel supply port to where I took the fuel line off, and the other end (fuel supply line) will go to the line from the filter.[/quote]
Correct the adapter IS the tee. The adapter should screw into the TB where you took out the supply line, then the supply line reattaches to the adapter. Then you can attach the gauge to the schrader port. The adapter can remain permanently installed since it has the schrader valve.
Also, the fuel line is usually bracketed to the transmission, so you may need to take it loose to get some slack on the fuel line, and then reattach the bracket after everything is put together.
- You must be logged in to reply to this topic.